Well the point of a film or media in general is more designed around entertaining the viewer and not always used to accurately portray what actually happened in the events in question.
This is why that a lot of films try to put small warps in history to keep it entertaining but at the sacrifice of some historical accuracy.
Since most movies often go by their own set of fictional characters that often don't have anything relevance to what actually happened during an event that plays out on the screen, directors have the freedom to put their own twist on how the events could have actually been carried out but still with the same result that we all know today.
But if the movie is more documentary based, constantly talking about dates and facts and not centred around the life of this character that never actually exsisted, then it is safe to safe that it is a pretty reliable source of secondary information.
A good example of a movie that is historically inaccurate would be "300" but yet it definitely provides in the entertainment department. Such inaccuracies as:
-Sparta was a city in Greece, not a country.
-In the battle of Thermopylae the Spartans were accompanied by other Greeks with their forces numbering between 6,000-7,000, not 300.
-The 300 "sire units" died rather instantly with no glorious blood bath, as the thousands of Greeks were no longer there to hold the line.
-They talked about the elements of "freedom" which were in fact absent in actual history, as Spartans were the biggest slave holders in that area.
And don't even get me started on the persian side of things.
So really don't always trust in what large, big budget, Hollywood movies tell you! As most likely it's not what your textbook will tell you. Stick to factual means of information like a textbook.
Hope that helped a little