A "General/Officer" in a medieval battlefield would almost always be a man of some kind of social standing, ie; a duke or count. Exceptions to this might be found in the condottieri mercenary companies that fought in Italy in the 14th century, who might have been led by an experienced veteran soldier, or low ranking knight.

either way, in the high middle ages most nobles, and therefore generals, were expected to be physically fit and able to participate directly in combat. /uPlinytheHipster made a good point on this here: https://redd.it/4uv4gs

We see this many times in recollections of battles. Richard I himself leading a charge at Arsuf and storming the beaches of Jaffa in the 3rd Crusade; Edward The Black Prince (a boy of only 16) on the front lines at the battle of Crecy in 1346; John of Gaunt, duke of Lancaster, leading his chevauche through France during the Hundred Years War; the Polish king Wladyslaw Jagiello is described as being in near mortal danger during the Battle of Tannenburg in 1410.

Of course, it wasn't always kings and royal relatives at the front of the lines. As I mentioned about Crecy, the young Prince Edward commander the vanguard not alone, but by his side stood the Earls of Oxford and Warwick, John de Vere and Thomas de Beauchamp (respectively).

For these men, the battle was right up in their face. At Crecy, Prince Edward is remarked as facing a very hard pressed assault on his unit, but his father, Edward III, refused to send reinforcements, taking on the famous "let the boy win his spurs" line.

There have, of course, been times when generals and commanders have stayed in the rear in an effort to better coordinate an assault. The issue with this is, and I'm going to go back to Crecy here, no battle plan survives contact with the enemy. This holds true especially in the middle ages. Unlike today, where we can simply radio to our LT's and Captains on the ground when something goes wrong, in those days commands had to be delivered personally.

At Crecy, King Philip VI wanted to form up a large, devastating cavalry charge on the English formations. To do so, he needed the army to stop moving and organize itself. Unfortunately for him, the sheer size of the French army and the slow speed of communications made that impossible. When one column of men would stop, another would keep moving. At this point it became a question of honor. Realizing that he had to begin battle at that moment, Philip sent his crossbowmen up to try to shoot the English. The longbow has farther range than the crossbow, so the poor Italian mercenaries were cut to pieces by arrows, and then trampled by a French cavalry charge. Thee French cavalry charged up the hill, into English arrows and were slaughtered. Those who reached the English lines gave a good fight, but the crushing blow of the charge had been lost, and the English had the advantage of the hill and eventually emerged victorious.

Another good example of a commander in the rear is Saladin at Arsuf in 1191. Saladin was not, from my knowledge, a front line commander. At the very least, he was not trying to be on this day. From reading over accounts of the battle that day historians have gathered that the Saracen army was trying to lure zealous crusaders out from behind their column of infantry.

Richard I had devised a march along the coast where his knights, the backbone of the European army, would be flanked on the west by the sea and the east by allied infantry and crossbowmen. The infantry would, presumably, be able to use their shields to protect the cavalry from Saracen skirmishers and the crossbowmen would be able to return fire.

A good enough strategy, but the prideful European knights aren't known for their patience. Richard's ability to instill discipline in his army is extremely commendable, especially as the rear guard (mostly Hospitaller knights) came under extreme pressure. Repeated requests from the H. knights to charge we refused, but eventually the Saracens got too close for comfort.

Two H. knights broke ranks and charged, which of course led to the whole rear to engage the Saracens. Richard was forced into this position but acted quickly, ordering a general charge which quickly routed the Saracen skirmishers.

Saladin had carefully planned the attack on the Crusaders, as he had done successfully at Hatin a few years earlier. Lure the heavy cavalry out, surround and kill them. The issue is they couldn't surround them, and in their efforts to force the knights to break ranks they got too close.

The crusader charge came up too quickly on the lightly armored and equipped skirmishers, and though Saladin tried his best to regain order over his soldiers, they were already starting to flee.

In medieval battle a commander's placement was largely up to himself, though lesser nobility would almost always be on the front lines. Their placement might benefit their command ability, but most orders and commands were delivered either by banners or messengers.

I brought up Tannenberg earlier, where the Polish king was hard pressed by Teutonic advances and, from the front lines, sent messengers to alert his allies to his predicament. they refused to come to his aid, the polish knight Mikolaj Kielbasa is recorded as stating that if they abandoned their flanks to assist him then it would do little good as they would soon become surrounded.

You ask what a battle might look like, and I would, based on these examples, state that it would look chaotic. From the frontlines or the rear, efficient command can run into numerous problems. Louis experienced his nobles not heeding his commands, marching on wards into English archer fire. Wladyslaw experienced the same disobedience, but it ended up benefiting him. Significance? Both nobles experienced the same issues with stubborn nobility, but from opposite ends of the field. Richard I was definitely a front line commander, and is remarked as having cut down many men personally at Arsuf and, later at Jaffa, was one of the first men to leap onto the beach from the boats.

Sources

http://deremilitari.org/2013/04/jan-duglosz-the-battle-of-tannenberg-or-grunwald-1410/ (De Re Militari is actually a fantastic website with dozens more primary sources on battles all through the middle ages. If you really want to buckle down and learn some more, head here)

http://web.archive.org/web/20101230001018/http://www.deremilitari.org/resources/pdfs/devries4.pdf (A description of the Battle of Crecy)

http://www.militaryhistoryonline.com/crusades/articles/arsuf.aspx (A decent, albeit short, description of the battle of Arsuf)

As far as books go, the book 'The Crusades: An Authoritative History of the War for the Holy Land' by Thomas Asbridge is a great read.

'The Hundred Years War: The English in France 1337-1453' by Desmond Seward provides solid coverage of the Hudred Years War

A little more recently published, 'The Battle of Crécy, 1346' by Andrew Ayton and Sir Philip Preston is a book I used for many essays on the time period.

For something a little more general, I recommend 'By Sword and Fire: Cruelty and Atrocity in Medieval Warfare' written by Sean McGlynn. It goes into pretty good detail or warfare in the middle ages in general, with extra focus on civilian casualties and general destruction. May not be exactly what you're looking for, but a good overall coverage of the period up to around the 14th century.