#1. There Is No Problem That Cannot Be Solved by the Application of a Sufficient Quantity of Pyrotechnics. Movie special effects departments are good at making things burst into flame or explode. If an automobile goes off a cliff, expect it to burst into flames without fail. If a siege weapon throws a missile, expect it to be a flaming one.
Thus the classic cliche of the historical movie, the fire arrow. While these weapons had some utility against flammable structure, they weren't actually very good antipersonnel weapons. If you have an arrow sticking out of your neck, the fact that you are also suffering a nasty burn is a secondary consideration. Wrapping bulky flammable material around an arrowhead is bad for air resistance, accuracy, range and penetration, and pausing to light the arrow slows the rate of fire. In clicheland, none of this matters, since fire arrows look good on the screen, especially if the battle is fought at night.
In the real world for most of history, fighting at night was something generals tried to avoid, since your troops were likely to get pointed to the wrong direction and start killing their friends. If you did fight at night, you wanted a stealthy sneak attack that avoided things that might spoil the surprise and make you an obvious target, like carrying flaming torches.
Timeline brought the cliche to a new level: a night attack with flaming arrows, siege engines and cannons throwing photogenic incendiaries, soldiers carrying torches, and a flaming moat. Why attackers don't just wait for the moat to burn out is not explained.
"We'll give them an unpleasant surprise" laughs Cardboard Villain #1 "NIGHT ARROWS." His archers fire a barrage of ordinary, non-flaming arrows. "AAARGH! NIGHT ARROWS" scream the terrified French opponents.
Flaming arrows are a useful resource for any director that has run out of plot or characterization
#1a; Medieval Napalm. Cheap and ubiquitous petroleum distillates are a commonplace of the modern world. Just pour a can of gasoline on the ground, strike a match, and WHHUMPP! Not so in the premodern world. Try this experiment: pour some vegetable oil on the ground and try to set it alight and see how far you get. There was some use of naphtha and naphtha based incendiaries, but they weren't easy to get, and more common in the Middle East than elsewhere.
In the counterfactual universe of Braveheart, William Wallace has easy access to large quantities of gasoline. At the battle of Falkirk, he apparently has a tanker truck parked behind the lines, so that he can wet down a broad stretch of the front-line as a death trap for the enemy. It is then set alight by flaming arrows, to set the enemy stuntmen on fire so that they can run around screaming while the flammable stunt clothing blazes merrily over their Nomex jumpsuits. Wiliam Wallace can also generally get his hands on fire starter whenever he wants to burn English soldiers to death in a cottage.
#2. Heroes Don't Wear Helmets. Against real world edged weapons, a helmet is your first buy and best value. Your head is right on top of your body where it is easy to hit, and a blow to the head will give you a bad day in a hurry. For the Hollywood hero, a helmet is an encumbrance to be discarded as soon as possible, so that the hero's face can be more easily seen and recognized. Unless it is desirable to wait until later to suddenly reveal that the armored figure is female, evil or somebody who we have already met.
#3 Men of Iron, Armor of Cardboard. Armor is surprisingly useless against most forms of attacks. Whenever the plot requires, arrows and sword thrusts will punch through armor with ease. This is related to:
3a. Braveheart Brigandine: This consists of metal plates riveted beneath a leather covering with a gap between the plates. This as flexible and easy to make, and virtually useless as protection, because any thrust will slide along the plate until it reaches the gap, slides into it, and kills the wearer. Its most perverse variant is the Braveheart Pajama Bottom of War: trousers with metal plates riveted to them with *large* gaps between them so the wearer can move. These gaps allow William Wallace to chop the wearer's legs off with ease.
3b. Studded Armor. Leather armor with decorative studs. This is designed to look like brigantine or similar armor to someone who doesn't have a very good idea what brigantine looks like. The studs offer approximately the same protective value as loose change in the wearer's pocket. However, the combination of metal studs and leather is very popular in bad historical movies, as well as the kind of bar where the patrons like that sort of thing.
#4. Real Men Don't Wear Dresses. Costume designers often fear that actual male medieval clothing looks like a dress and will confound the gender expectations of their audience. Medieval tunics and robes can end up morphed into short jackets, smoking jackets (Knight's Tale) and dusters (Timeline). Hosen tends to turn into pants (Knight's Tale) and trousers (Branagh Henry V)
#5. Bad Hair. The modern filmmaker is really reluctant to put their characters, and particularly protagonists, in hairstyles they think their audience will find unflattering. Thus the unmedieval bangs in Timeline and the '30s mustaches in the Errol Flynn Robin Hood. Olivier's Henry V and The Warlord show rare courage in putting their heroes in appropriate haircuts that look unflattering to many modern eyes.
#6. The Antagonists are Eeeeevil. Particularly if the protagonists are killing large number of the antagonists, having completely evil bad guys helps avoid any nasty moral ambiguity to the body count. Cardboard Cliche Villains don't hesitate to promiscuously slaughter random civilians (Timeline), rape and kill women (Braveheart), not necessarily in that order (The Messenger) or toss babies into the fire (Alexander Nevsky)
6a. And Yucky. Cardboard Villains can be unattractive in other ways, to make them even lesss sympathetic. The Edward II in Bravheart is a weak and mincing effeminate. The historical Edward II was physically strong, well formed and vigorous, whatever his moral faults. The Commodus in Gladiator was a dark, puffy faced dissolute. His historical model was an athletic blond. Alternatively, the Cardboard Villains can have bad teeth or other deformities. (The Messenger)
6b. Droit de Seigneur, the legal right to deflower unwilling virgins would have been a great way to be a Cardboard Villain if the institution had actually existed in the Middle Ages
#7. Protagonists can do no wrong. If a historical protagonist has actually made a belt from the skin of an opponent, or carried out a campaign of burning and pillage aimed at civilians, this will not appear in the movie (Braveheart)
#8. Amazing Portable Siege Weapons. Enormous multiton siege weapons can always be deployed from somewhere else over medieval roads to where they are needed in whatever time is required by the plot (Timeline)
#9 Random Melee. Some modern fight choreographers like to show the chaos of battle by scattering fighters of both sides randomly about the field in a series of mostly single combats. (Braveheart, Branagh Henry V, etc, etc, etc.). If you have gotten yourself into this kind of situation on a medieval battlefield, you, your companions, and/or commander are incompetent and will probably be dead in a few minutes. If you're doing it right, you are standing in good formation with an ally on your left and your right, and you won't break formation until your enemy is fleeing in rout, if then. Alexander Nevsky is one of the few movies that comes close to getting this right.