This question strikes to the heart of military theory in the long 18th century: what makes a good soldier? And by extension, what makes a good army? To long-18th century (1700-1815) minds, the answer was experience. Experience made men calmer under fire. It made them more fit for long campaign marches and sleepless nights, it made them more hardened to cold camps and poor food, and it made them more adaptable to changing campaign or battlefield conditions. It made them more reliable to their officers and less likely to break or freeze on the battlefield. It also made men simply better at handling their weapons, achieving either the three shots a minute favored by the British or the inexorable advance of the bayonet favored by the French.

Obviously, this isn't easy. How you got experienced men instead of dead men relied as much on luck as it did skillful handling. Training also wasn't considered as reliable or as effective as it is today. The British soldiers who fought at Bunker Hill, for instance, had spent a good deal of their time in garrison drilling and firing at targets floating in the harbor, and at the time of the attack they were likely the single most extensively trained soldiers in North America. And yet, when faced with the withering American volleys, they froze, clumped, and broke, refusing to listen to their officers. This is just one example of a doctrinal belief that training was itself insufficient to create dependable soldiers, and instead the sieve through which recruits became soldiers was experience. Training at the time was, of course, very different than training today. I expand a bit more on this idea here but for now it's only important to say that training was done on a regimental basis or on the march, as time afforded, after an extremely basic familiarization course at recruitment depots which taught the basics of march and maneuver - period manuals go into such detail that they often specify the physical mechanics of taking a step forward - and the manual exercise, or the practice of loading and firing their firelock. It was a fairly common practice in the 18th century to fire very few practice shots, instead running through dry-fire practice without actual powder or shot.

By some period estimates, it took as long as two years for a man to become a solid and dependable line infantryman. For cavalry and artillery the estimate was different, and artillery remained for a long time the only branch that had anything like a regularized training program for their men, usually at private or state military academies. Experience, of course, doesn't mean solely time in combat, but time on campaign, time in-service, time drilling and all the other mechanics of war. All of these are important, of course, but experience with the chaos of battle was by a wide margin the most important. Nothing can prepare a man who'd never before been shot at except the act itself.

But let's say for instance we have a hypothetical soldier, who'd volunteered or been recruited in the early 19th century, and by chance or skill he's managed to survive long enough to reach that temporal switch that changed him from "recruit" to "soldier." Chances are that he would not be alone, and that his survival was his own doing. National recruiting varied by country, but in British and American practice whole regiments could be raised at once, and their manpower maintained through regular recruiting at home. This means that either our soldier was recruited at the time of the creation of the regiment or sent along after it had already been formed. In the first case, some of the original members would have survived along with him, avoiding death in combat or (much more likely) death by disease. As more and more new faces filter in from the recruiting back home, the core of the regiment would more and more be surrounded by untested, inexperienced men. The veterans, then, were partly responsible for teaching the new recruits the kinds of things that all soldiers are expected to know; not just how to handle weapons and march, but where to pitch a tent, how to forage for food, how to maintain their clothing and lug along the camp equipment and personal dunnage. This doesn't mean that our hypothetical soldier is recognized in this role, much less that he's paid for it, but it is indisputably a part of being an experienced soldier. The veterans also serve a role in instilling the sense of the regimental or battalion esprit de corps, the sense of the organization itself to its members. Stories of old battles, old members, rumors and tales about the officers or men of the ranks, their pranks and foibles and mistakes and triumphs. They would teach new men to respect the guidon and the regimental colors, to be instituted, essentially, into the mysteries of a select group of men with a specific history.

In battle, these things matter. Studies of battlefield psychology continuously suggest that shared affinity is a major factor in a soldier's behavior, and avoiding looking like a fool or a coward in front of people you know and like - if not love - is a strong motivator. To put it even more crudely, you're more likely to listen to an officer if the other men in the regiment do, and you're more likely to run away or stand and shoot or charge with the bayonet if the men around you are doing the same.

So far this has mostly focused on camp and campaign life, and thee campaign communities are vital to understanding warfare, but you asked specifically about battle. The wrinkle here is that individual soldiers, no matter how experienced, would not be put to use in any specific sense (apart from, obviously, officers), but groups of men might be. Regiments, brigades, and battalions themselves have experience and a reputation for behavior, which is tied to their behavior in battle, the temperament and reputation of their officers, the conduct of their men, and about ten thousand other things. As a member of an experienced regiment, our hypothetical soldier might be put on the flanks of a formation where the line was vulnerable, or might be put in to lead a charge through a breach in a besieged town, or might be given an order to hold until relieved against enormous odds. Military history is littered with examples of this. Of course, individuals within those regiments might be singled out for promotion or decoration, or be "mentioned in despatches," to the commander of the army as a whole, and their names might even end up in newspapers back home.

Another way an experienced soldier might be utilized was as a volunteer for a particularly dangerous job. Scouting and raiding, cutting-out expeditions to capture enemy ships or stores or to take prisoners were often conducted by asking for volunteers to fill up ad-hoc detachments whose purpose was to perform a specific task and then they'd be sent back to their original units. This was especially common practice in the frontier regions of the War of 1812, where militia volunteers often made up the bulk of cross-border raids. One of the more notorious of these types of tasks was what was called the "Forlorn Hope," a detachment of volunteers formed to be the first wave through a breach in a fortified position. It was intensely dangerous and was often rewarded merely by a bump in pay or social rewards.

As for advantages, they tend to be somewhat nebulous. Our soldier would likely know that the combat doctrine of their officers was sound (or not), and that safety in battle was often counter-intuitive. The men storming Breed's Hill, for instance, stopped their advance because they believed firing at the American line was safer than advancing with the bayonet, despite their officers ordering them to charge. A veteran would know better; bayonet charges were often decisive, one way or another. He would likely know that an orderly withdrawal is better than a rout, and how to best achieve it. They'd know the quirks of their own weapon and how to keep the regimental colors in sight as the powder smoke and noise and cries of the wounded and dying filled the air. They'd know a lot of things that are difficult to quantify.

To make a long story short, veteran soldiers are far more useful than simply as effective cogs on a battlefield wheel. They are lore masters, cultural authorities, cooks and grooms and mechanics and tailors, they are foragers and camp engineers, and they create a vast multifaceted network of social shorthand that creates a unit's shared history and their chain of command. They create and sustain the shared experience that makes the difference between a mass of men and a cohesive military unit, both on the battlefield and in-between.

I’ll take a stab at this, though I should preface by saying I am by no means an expert. I’ll lean on John Keegan’s “The Face of Battle” here, because he does dip into this particular question at length at various points in his chapter on Waterloo. Before I continue, it’s worth noting two caveats. The first is that, as far as I know, there is a relative lack of evidence for first person accounts of the average “line infantryman” of the Napoleonic Wars. Most of our memoirs, journals, etc come from officers, given that the majority of enlisted men would not have had significant access to the materials needed to record their memories and quite possibly would not have been literate enough to comprehensively do so. As Keegan notes, “Alas, the motivation of combat, individual or collective, of the private soldier of this period is almost impossible to analyze, for we know so little about him.” Unfortunately there really is no brilliant book like E.B. Sledge’s “With the Old Breed” (an account of the average Marine infantryman’s fight in the Pacific theater of WWII) that I am aware of to give us a clear picture of an infantryman’s lot. The second caveat is that armies from various nation-states fought differently and thusly the experience and usage of a Napoleonic infantryman was not universal. The French, for example, famously used columns as an attack formation quite extensively. The British, on the other hand, relied much more on line formations and firing by platoon, using “superior musketry” to win battles. Austrians, Russians, Belgians, Prussians, etc all used their infantry in various different ways despite broadly adhering to the overall tactical understandings of the time period. Regardless, I’ll try and give the best answer I can here.

You are a veteran line infantryman. With this statement, we are making a number of assumptions. You are not a light infantryman, rifleman, or the like. You are trained to the standard your army deems fit for combat. You have also seen combat at least once, but, as “veteran” usually implies, probably more than once. You have been fired on and have fired on others. You are somewhat prepared for and/or inured to the noise, lack of visibility (it would have been incredibly smoky on a Napoleonic battlefield), and smell of battle having experienced it before. Furthermore you are hardened to the physical exertions and realities of Napoleonic campaigning: long marches, limited rests, limited food supplies.You have experience, but, for the purposes of this question are not a NCO and therefore have no obligations for leadership.

Immediately advantages over green troops, which I will assume those freshly brought forward from training with no combat experience, should be apparent. One of the most important is undoubtedly experience with the physical sensations and psychological impacts of an actual Napoleonic battlefield. To quote Keegan regarding Napoleon’s infantry at Waterloo:

“The Old Guard contained none but veterans of long service; but even in the line regiments the majority of men had seen action, and had handled their weapons under fire. They would thus have learnt not only to bear the fatigues of campaigning but would also have been familiar with the two other most oppressive characteristics of the battlefield: smoke and noise.”

This experience with combat would make it much easier for you to endure the chaos of battle and much less likely to break and flee when in contact with the enemy. And trust me, there was a lot of smoke and a lot of noise: the whistling and booming of artillery, constant crackling hum of distant musket fire, the fire of those around you. According to Keegan, “the noise was so loud and continuous that you could hardly hear what was said by the person next to you.” The ability to follow orders and stand firm in these conditions was enhanced beyond training only through enduring the experience itself. Add to this the ability to process trauma that can only be found in battle. Men dying, often in violent and horrific ways (ripped apart by a howitzer shell, for example), has an undeniable psychological impact, but it is widely agreed that repeated exposure to such violence (as the aforementioned E.B. Sledge notes in his own book, especially regarding Marine actions on Peleliu) breeds a certain sangfroid when confronted with utter brutality. This in turn also implies to inflict death on others. Keegan notes that veterans possessed the surprising and almost incomprehensible capacity to engage in, “offering and delivery of death over distances at which suburbanites swap neighborly gardening hints, their letting of blood and infliction of pain in circumstances of human congestion we expect to experience only at cocktail parties or tennis tournaments.” Killing, in short, seems to become somewhat easier for soldiers with exposure to such vile work.

The psychological difference between recruits and veterans was often quite stark in battle. Green Dutch-Belgian troops, for example, were the bane of Wellington’s army during Waterloo for their tendency to “flagrantly shirk” responsibility in combat, making any effort to avoid the noise and chaos of battle. In short, when exposed to the slightest amount of pressure from enemy fire, the Dutch-Belgians had a tendency to try and run away. Veteran infantryman, such as the 92nd Highlanders, showed no such compulsion to flee despite intensive suffering. As an extreme example, the 27th Inniskilling, veterans of the Peninsular War, held formation almost unto complete annihilation under artillery fire, throwing their dead out of the square during breaks in between bombardments and closing ranks, a feat that men largely inexperienced with the brutality of Napoleonic combat would have been hard-pressed to achieve.

Tactically and training wise, there is probably not much more you as a veteran line infantryman would know than your green counterparts, other than simply having more time to learn and practice. Perhaps you know a means to shoot slightly faster by carrying your cartridges differently or a better trick to fend off cavalry or another man’s weapon with your bayonet, but it doesn’t change much, given that the infantry tactics of the time don’t allow for single combat flourishes. Per Keegan, “Single combat… was at Waterloo almost exclusively the affair of cavalrymen.” As cinematic and action-packed as Sharpe may make Napoleonic infantry combat appear, it was rarely so. Keegan goes on to note that, barring a few specific instances (namely the extremely close quarters defense of the strongpoints of Hougoumont and La Haye Sainte), there is very little incidents in which infantry opponents actually fought bayonet to bayonet at Waterloo, thus rendering individual combat skill a marginally useful veteran quality when compared to the ability to stand through hellish conditions unbent and unbowed.

The one significant tactical advantage you might have over a recruit is the understanding of fire discipline. Particularly in square, Keegan notes, the need for holding fire against cavalry until the absolute last moment was paramount for success. Firing too early would result in the cavalry being able to perhaps harm the square, but firing at the right time would break a cavalry charge dead in its tracks. Similarly, you would know not to throw your fire away early when confronted by enemy infantry and would be more capable of adhering to the instructions of your officers in this regard. You might also know how better to deal with certain types of foes. For example, experienced infantrymen in regiments were often heard to comment to others on how to aim at different targets. “They are in armor, fire at the horses” was a veteran's cry heard among the 40th when cuirassiers attacked their square during the frantic cavalry charges at Waterloo. Still, in large-scale formation based combat, where weight of bodies and fire counted more than the individual combat skills of each soldier, technique gains would have had a minimal impact on proceedings.

As such, the enduring advantage of veterans remained largely practical and psychological. Men who had experienced combat before were simply better prepared to deal with combat again. You are more efficient at fighting and killing, not because of any particular tactical insight gained, but rather because you are far less likely to panic, disobey orders, shirk, or otherwise be shaken in the midst of battle. You are less likely to balk when advancing over dead men in columns, for example, or become inconsolable when a cannonball annihilates the man next to you. Furthermore, you have the proven psychological and physical tenacity to perform, as Keegan explains, the main duty of the Napoleonic infantryman: holding or taking ground despite whatever is thrown your way. As Wellington himself once told Halkett, who was asking for relief for his infantry brigade, “Tell him what he asks is impossible: he and I, and every Englishman on the field, must die on the spot we now occupy.”