Depends on when and where. Pre-20th century, after-all, covers over 2500 years. While I've not read anything explicitly stating it, it seems to me command style changed with the size of the army and battlefields.

In Classical Greece, commanders could basically only lay out a battle plan before it starts. Once all the sub-commanders have their orders and the battle starts, there's nearly no control whatsoever. This is due to the fact that the general fought as a hoplite on the front lines. So it was very hard for him to know what's going on and issue new orders. The best he can do is guide his own detachment according to the pre-set plans. With battlefields being relatively small in size and usually no reserves to speak of, it didn't really matter that much I suppose.

Just prior to Alexander, generals on horseback began to appear in Greece. But they still very much fought on the front with the cavalry. So they had more control and could respond better to situations around the battlefield and far more quickly. Alexander at Gaugamela is a well known example, but I like Pelopidas responding to battlefield situations at Cynocephelae even better.

The successor states probably eventually adopted having the commander and his bodyguard cavalry sit in reserve, so he's busy commanding instead of busy fighting. And this way he knew where to send the reserves because watching from slightly behind the lines. If the successor states didn't do it, certainly by the Roman Empires that's what was done.

It's not all that linear though. For example at Hastings long after the fall of the Western Empire when armies were smaller again, Harold was on foot in line with his housecarls, similar to the hoplite style, with no control over what his brothers did with their contingents. William on the other side fought with his Norman cavalry (though I'm not sure if he was in reserve or not, it seems he was pretty active). He had to take off his helmet to show everyone he was still alive at a point in the battle, and led a few attacks.

There are other variations too. Supposedly Mongols like to set up their commands on a hilltop instead of with the army and issue orders with horns, drums, flags, signal arrows, etc (forgot which book I read that in, was it Genghis Khan and the Making of the Modern World?) Similarly, by late Sengoku the Japanese prefer to place the commanding general and his personal troops on a hilltop slightly behind the fighting where he can see what's going on and issue command to move forward. And on the other side of the world this was also done in European battles such as Naseby, as both King Charles and Fairfax were slightly behind the battle on top of ridges, though Charles apparently didn't commit until it was too late, so ended up not committing at all.

By the 19th century (at the latest) army sizes and battlefield had grown so large that it was very difficult (or impossible) for the commanding general to see the entire battlefield. So a lot of time the general have to act on reports of aides and messengers. Since messages travelled at the speed of a horse, and you aren't seeing it with your own eyes, it could get pretty confusing. See examples in this thread.

An interesting case study in the real-time control of the battlefield is the famous Charge of the Light Brigade.

The British commander was Lord Raglan, who was standing on top of a hill and able to see the whole battlefield. In front of him, at one end of a long valley was his brigade of Light Cavalry. At the other end of the valley was a well-defended enemy Russian artillery battery.

Behind Lord Raglan one of the Russians' other artillery batteries was in retreat. Raglan wanted to order the Light Cavalry Brigade to chase after them and seize their artillery. With no radio, he had to send a runner with a note which read Lord Raglan wishes the cavalry to advance rapidly to the front, follow the enemy, and try to prevent the enemy carrying away the guns. Troop horse artillery may accompany. French cavalry is on your left. Immediate.. This was accompanied with a verbal order to attack immediately.

Now there was a crucial mistake would not have occurred with modern technology. The Light Brigade only had knowledge of one Russian artillery unit, the well-defended one at the other end of the valley they were standing in. The Brigade set off down the valley. The runner suddenly realised the mistake, and chased after them to redirect them, but was shot down by the Russians. There was no mechanism to contact the unit and call them back. The Brigade did successfully and bravely seize the artillery but were surrounded and taken out by other Russian units.

Lucan, the commander of the cavalry, later claimed that he had asked the runner which guns were being referred to and the runner had unhelpfully swept his arm across the whole battlefield, where only those at the end of the valley were visible.

My are is First World War and just before, which isn't technically within the scope of your answer, but I'll go ahead and answer anyways because I think you might find the answer helpful.

First, let's pick apart your question a little bit, because I think that in doing so, we can start to see some of the ways in which battle and the concept of "a battle" had changed and was still changing by 1914. How much real time control a commander involves two things, in my estimation: a) the ability to get accurate information regarding the course of battle at any given moment and b) his ability to relay to combat forces his commands. The fulfillment of one of these conditions does not necessarily mean the second is automatically fulfilled. On the other hand, both are closely related and often rely on the same technologies, or lack thereof.

Moving on, some people in the thread have mentioned that the answer depends, based on what period we're talking about. This is obviously true, but it also depends hugely on what level of army organization you're looking at. /u/ParallelPain answered with regards to a Classical Greek battle, where you could theoretically watch the entire battle from a high enough vantage point. One of the biggest trends in nineteenth century military history is the expansion of armies with mass conscription and states with the organizational and material wealth necessary for fielding armies of millions. By the outbreak of the First World War, we've got not only regiments (the basic unit of infantry armed strength), but also divisions (which unite infantry strength with artillery and cavalry force, as well as logistical support), but also corps, armies, and even army fronts comprising two or more armies.

This also forces us to acknowledge that by the 20th century and the First World War, even the concept of a "battle" has to be looked at and played with a little bit. Going back to Greece, a battle might last a few hours. Skipping over quite a few centuries of military history, by Gettysburg we have a three day battle. But these are still unquestionably 'battles.' By 1914, we see weeklong engagements like Tannenberg, the Marne and Komarow. These single 'battles' are titanic when compared to single-day infantry clashes in Classical Greece. In fact, these 'battles' may be more accurately described as a vast network of smaller battles all being fought with regards to a single operational objective, e.g. "capturing X town." Then we have Verdun and (moving forward) things like Stalingrad, which challenge the conception of a 'battle' as a unit of analysis even further.

All this was a very roundabout way of saying that commanders at different levels are going to have different means for a) observation and b) control based on the number of men (and cavalry detachments, artillery batteries, and supply columns) they're commanding and the geographical and temporal scope of the 'battle' they're overseeing.

I focus on the Austro-Hungarian Army, so we'll use them as an example. Each turn of the century army had its institutional peculiarities, but roughly, this should suffice in general. I'd love for people more knowledgeable about specific armies to chime in and provide some context.

Now for the actual answer. Let's start at the top. The truth is, radio and telephone communication at the outbreak of the First World War were nascent technologies and their application to ground combat was still being figured out. But with the larger units of organization - armies, corps, divisions and even regiments - the telephone and telegraph were being used quite well by the Austrian Army. In 1914, each division had its own signals detachment, armed with a portable telegraph set and several kilometers of telegraph/telephone wire that could be unrolled and laid at the will of the commanding officer. Communications travelling from division commands up to corps and army headquarters were quite regular. The Austrian Army meticulously (by necessity) recorded a lot of information on each telegraph: when the message was sent, where it was sent from, which command was sending it and to whom, but also when it was received, when it was read, by whom it was read and to whom it was passed along. These messages varied in the level of detail, but the most common message was a "Disposition" and sought to give the corps or army headquarters where the division and its constituent regiments were located and what they were doing. For example, a telegraph I have a facsimile of from the Vienna War Archives reads, "have arrived in the Cossack barracks [in the town of Zamosc]... and captured assorted war material, including a munitions wagon, 2 portable kitchens, as well as 2 guns on the road towards Lubelskie." This was sent from a divisional commander to his corps commander and is typical of the type of communication being sent up the chain. As you can see, this message and others like it leave a lot to be desired. Telegrams were also being sent down the chain of command, with corps commanders telling divisional commanders where their neighboring units were, how their mission objectives were progressing and whether enemy activity had been reported in their sectors. These telegrams were essentially aimed at giving commanders an idea of where forces were on the battlefield and what they were doing. Divisional and corps commanders also relied on hand-drawn dispositional sketches showing where units were with regards to certain objectives and landmarks. What these sketches made up for in detail that telegrams couldn't convey, they lacked in speed. Maps had to be ridden (by bicycle, horse or car) to the pertinent headquarters. This could take anywhere from ten minutes to several hours depending on the state of roads, how well the area was known to the different commands, and the combat and traffic situation of those roads.

This has all so far been about how commanders got information on where their troops were. As you can see, it would take a powerful intellect with massive amounts of training to put together a cogent picture of what was going on. In engagements which were lasting several days to a week, the time constraint was, as with all combat, very important, but commanders could usually get a telegraph message from a subordinate within an hour, from the moment the subordinate decided to send the message to the moment a transcribed message was handed to the superior officer. This, combined with illustrated disposition maps and the spoken word of messengers, was no Blue Force Tracker, but it worked reasonably well.

As for sending commands and trying to affect the flow of battle, commanders relied on the same sorts of technologies. Written commands were transferred via telegraph to subordinates in the same way dispositions were reported going the other direction. For example, one telegram I have from II Korps to 13th Division command tells that commander: "both infantry brigades, the artillery brigade and artillery HQ are to immediately assume a disposition on the northern egress road from Zamosc and to establish connections with 25th Infantry Division." Like reports being sent up the ladder, commands being sent down the ladder could be expected to be read by the pertinent subordinates within an hour.

The telegraph I chose helps me segue into the next issue, that is, sub-divisional commands and reporting. By 1918, individual regiments would be armed with telegraph units and plugged into the communications network, but in 1914, all communications at the sub-division level (regiment and battalion) were done on paper or by messenger. The Vienna War Archives is literally full of these tiny bits of paper, about three inches by five inches, with written dispatches on them. The blank dispatch cards were printed en masse in perforated card books so that the officers using them could save time by plugging in the information into pre-printed spaces. In typical Habsburg bureaucratic function, there are spaces for every bit of pertinent information: sender, receiver, time sent, date sent, area sent from, etc. Speaking to the paramountcy of the horse and rider to sub-divisional command is the spot on the dispatch card where the writer is asked to circle one of the following: "trot" "gallop" or "trot and gallop." These messages took significantly longer to relay, depending on the geographical proximity of the sender and receiver and the intensity of combat in the area. Of course, many messengers went missing, were captured or killed and many messages never received, often resulting in lives lost. Hand-drawn maps were also very important at the sub-divisional level, especially as a means by which regimental commanders showed divisional commanders where their units were located.

Finally, at the very nitty gritty level of sub-regimental command, what we would consider "combat", flags, written notes and messengers had to be used. Since radios wouldn't become standard until the Second World War, battalion commanders had to rely largely on flag signalling and messengers to get their commands into the hands of lieutenants. At the beginning of the war, each platoon (60 men or so) had a 3-man signalling section: a patrol leader, an observer and a signaller. The signaller had two flags and used these to relay observations and reports up the chain. At the company-level, the Austrian Army experimented with field telephones even at the beginning of the war, but they were not standard, especially in the context of highly limited financial resources prior to the First World War. These telephones were operated using Morse code and were carried along with about a mile of telephone wire.

Sources:

Lucas, J.S. Austro-Hungarian Infantry, 1914-1918. Almark Publications, 1973.

Vienna War Archives, independent research

Bear in mind that it would be much easier to control your forces pre-gunpowder era than later, and with smaller army sizes. A battlefield with cannon and musket fire echoing across it would be much harder to communicate with, say, horns or drums, in comparison to if these horns were used in a pre-gunpowder era battle. Similarly, with smaller army sizes, you'd have less of a problem conveying messages from one side to the other, and overall you'd be able to keep an eye on the entire unfolding. In terms of "real time control", you also have to account for the delay between messengers getting messages across, where they could be delayed by opposing forces, or simply due to the time taken to traverse the field on horse or by foot. A basic battle plan would be in place but, as all battles go, would probably be modified as it progressed - you'd have a rough formation idea at the beginning, as well as certain rules for engagement/retreat, however it also relied on the best judgement of the commanding officer (who would relay his commands down through the hierarchy) or, if he wasn't available, the next person down, and so on.

There is a lot to this question, and answers will vary considerably depending on the era and the army. In the Napoleonic era, generals had their ADCs on horseback, typically in a "string" taking turns making runs out to relay orders to individual units. Aides were typically very trusted appointees who were part of the General's "military family" or retinue and very often a son or son-in-law, so in some respects they were expected to know very intimately what a general was thinking. They were often entrusted with scouting missions-- for instance, a Corps commander would send a staff colonel off with a fighting patrol of a squadron or two of light cavalry to take prisoners and see what he could stir up. On a lower level, a brigade commander could put his ADC, a captain, in charge of a light company and two grenadier companies from one of his regiments and have them overrun a troublesome artillery battery. Both these examples come from Col. H.C.B. Rogers, "Napoleon's Army" (1982), which has a rare couple of case studies of Napoleonic battles on a small scale. The memoirs of Baron Marbot also contain a lot of info about the role of an aide and light cavalry officer. The system relied on the skill and initiative of subordinates as well as the ability of a senior officer such as a army or corps commander to assume direct control over a developing situation. It helped that armies were relatively small and fought in small areas compared with 20th century armies. As Napoleonic armies swelled ever larger with masses of conscripts, and the best of the generals were killed off one by one, command and control grew more difficult and the tactics devolved into bludgeoning contests like Borodino or Wagram. For a details of how a battle plan could get entirely too elaborate for green troops and inexperienced officers to follow, "the War of 1812 in the Old Northwest" by Alec Gilpin (or most other accounts of that conflict) have plenty of examples. One case in particular is the Battle of the Rapids which occurred during the siege of Fort Meigs on May 5, 1813. I could write a detailed account but the basic details are available in plenty of sources in print or online. It illustrates perfectly what could happen when a commander lost command and control during a battle taking place over a wide, wooded area--and General Harrison was a professional soldier who had been schooled as an ADC for General Anthony Wayne fighting over the very same area. Basically, once a situation was developed it came down to the initiative of field officers to react or rally their units. Militia officers and their men weren't necessarily bad troops, but they had to be kept close in hand or risk disaster. Generals could affect a situation if they were able to gallop into the midst of a battle and rally or coordinate troops, but they ran the risk of getting shot, which frequently changed the game if their subordinate was not as energetic or skilled as they.

In the Franco-Prussian War the command and control system relied more on the independence of low level commanders on the battlefield, with von Moltke the Elder organizing the larger scale plans of battle. The idea was von Moltke could command the strategic movements of the armies, but allow the lower commanders who fully understood the ground situation to make the tactical decisions.