For a few reasons, you're probably never going to see it because (as /u/EyeStache pointed out) we don't know how they fought since none of the sources explicitly tell us (they assume the audience knew) but I feel modern audience has been so conditioned to view ancient/medieval battle in a certain way. No director will attempt to be 'realistic' as the audience would be confused, it would take a very skilled director with the pull to get it shot his way and willing to spend considerable screen time to 'get it right'.

Goldsworthy has a model of ancient combat that stresses the psychological aspects of warfare. That suggests that combat was pretty terrifying (in an age without Geneva Conventions, antibiotics, painkillers) and few would be willing to just charge straight into an enemy formation bristling with sharp pointy objects. So one would try to fight in a formation - be it a rigid Greek phalanx, a slightly more flexible Roman cohort, or even a loose Gallic tribal force - to ensure your flanks are covered. Certain men in the unit, be they huscarls, centurions, generals, will be the main motivator for the unit. They're the guys who'll whip the men into action, who'll lead the assault, who'll be fighting to kill their opponent. Most soldiers will be more intent on fighting to survive, concentrating on protecting themselves rather than killing their opponent. After several minutes of fighting both sides would tire and pull back to rest and to psych themselves up for another round. Rinse and repeat until one side takes too many casualties or loses the will to continue fighting (or both) and then panic takes effect and the unit dissolves. Its a good model and one that can explain how battles can take several hours yet the victor only comes out with a handful of casualties (even treating casualty numbers with some grains of salt).

Lastly, the majority of the killing was done when one side broke and ran. They'd abandon anything to slow them down like weapons and shields which would leave them defenceless and vulnerable when struck from behind. But there's hardly anything 'heroic' in showing Mel Gibson cutting down fleeing English soldiers without mercy.

Unfortunately, you're not really going to get a satisfactory answer to your question, because - with very few exceptions - we don't really know how mass battles were fought. A lot of the source material we have on battles doesn't describe formations or troop density or tactical manoeuvres or anything like that with any granularity, so we can't accurately recreate them on film.

That said, though, there are some films depicting Greek and Roman battles that might be more accurate than others, but that's unfortunately far out of my specialization, so another user will have to pick it up.

Everyone would do well in remembering that we require in-depth comments in this subreddit. Just throwing out a title of a movie and a short sentence or two is not enough. Explain what why it's accurate (sources please!), show examples and walk us through it.

The scene depicting Gaugamela in Oliver Stone's Alexander is pretty great in a fair degree of it's details - or at least in terms of the depiction of the account in Arrian, and of the Macedonian phalanx, and ancient warfare in general. There are several factual errors surrounding the battle and film itself, but if you were after a depiction on film of a massed ancient battle, there aren't many better.

In terms of why this is successful - details such as the terrifying but ineffectual scythed chariots match up to their description in Arrian, and pretty much every other time they are mentioned anywhere - they are frightening and terrifying constructs, but they rarely seem to have any effect on the overall outcome of a battle (as noted in Phillip Sabin's Lost Battles, among other places).

How accurate it actually is, is probably impossible to say, but it at least depicts the source material reasonably well, which is often about as good as you cab get with ancient warfare.

The battle scenes from Kubrick's Spartacus are an interesting counter-example. The Roman Quincunx formation is still a matter of debate (one I suspect is solved by massed battles looking rather more tentative and piecemeal than generally depicted on screen), but Kubrick's solution to the puzzle of a checkerboard army formation (i.e., "wouldn't that leave massive holes?") was to have them march into position in Quincunx, and then deploy into solid linesm which there is no evidence for.

As others have pointed out, no battle can be truly realistic as we don't have sufficient information to say precisely how they were fought, but we do still have enough information to say with a high degree of certainty that, in Western warfare, the chaotic Hollywood style of massed armies mingling and fighting one on one battles didn't happen. Instead, sources emphasize the need for a densely packed formation that fought together.

For instance, when fighting the Colchians, Xenophon writes:

In the first instance, the Hellenes drew up opposite in line of battle, as though they were minded to assault the hill in that order; but afterwards the generals determined to hold a council of war, and consider how to make the fairest fight.

Accordingly Xenophon said: "I am not for advancing in line, but advise to form companies by columns. To begin with, the line," he urged, "would be scattered and thrown into disorder at once; for we shall find the mountain full of inequalities, it will be pathless here and easy to traverse there. The mere fact of first having formed in line, and then seeing the line thrown into disorder, must exercise a disheartening effect. Again, if we advance several deep, the enemy will none the less overlap us, and turn their superfluous numbers to account as best they like; while, if we march in shallow order, we may fully expect our line to be cut through and through by the thick rain of missiles and rush of men, and if this happen anywhere along the line, the whole line will equally suffer.

(Anabasis 4.8)

Verbruggen, in The Art of Warfare in Western Europe During the Middle Ages, cites a large number of literary and poetic texts that point out the dense and tightly packed cavalry and infantry formations. He notes in particular that Raas of Gaver's advice at the Battle of Worringen that they thin their ranks in order to match the width of the enemy formation was considered by the chronicler Jan van Heelu to be a tourney tactic not suited to battle. Liebrecht, lord of Dormaal was of a similar opinion and is recorded as having said

Thick and tight!

Thick and tight!

Let every man press up stoutly to his

neighbor as close as he can. So we shall certainly win

glory today!

In contrast, the archbishop Siegfried's army advanced in a disorderly fashion, all his troops virtually in a single line. As a result, while the archbishop's army was able to outflank their enemies temporarily, they were unable to break the formations of the Brabant army and were ultimately defeated when a company of foot and horse (the peasants and knights of the Count of Berg and the civic levy from Cologne) arrived and took the archbishop in the flank.

With regards to the battles in Braveheart, they have only the vaguest connection to how the battles are recorded as being fought, and fall into the Hollywood melee trap. Stirling was a case of foot soldiers cutting off a portion of the English army from the bridge they were crossing, trapping them in a loop of the river and systematically killing them, while the main English force was largely without archers and was demoralised, leading to their retreat. Falkirk was a case of dense, circular infantry formations resisting a cavalry charge but ultimately being taken apart by archery and cavalry as a result of their tactical immobility.

In short, Braveheart doesn't even get the most basic aspects of the battles correct, let alone how they might have been fought.

I'd suggest you check out the film waterloo. The movie used 15-16 thousand soldiers from the soviet unions army as extras in the battle scenes. It in a lot of ways a reenactment of the battle. They had engineers prepair the battle area by getting rid of hills, planting 5000 trees, and laying out miles of roads, planting rye, and wild flowers. They uses irrigation pipes to create the mud of the battle. The 16000 extras were drilled in 1815 battle formations and how to handle bayonets, sabres, and cannons. The troops would function as units, and orders were given to the officers by the director to try get authenticity. So well its still a film at the end of the day there was a considerable amount of care put in try to keep it authentic as possible