Fight Scenes...

[Yora]

Fighting in lines or shield walls works because the whole is greater than the parts.

And this is something pretty much every movie ever gets completely wrong. I can't really think of any movie right now in which a big battle without firearms is shown with any degree of realism. The only scene I can think of that comes close is in The Two Towers when the orcs are breaking through the big door of the castle. What the defenders inside the door are doing comes pretty close to how battles were actually fought. Using the remains of the door as big shields for the guys in the front, and the rows in the back bracing them from behind, while they try to poke at the orcs outside with spears through holes in the wood. And the orcs are trying to push the defenders back through sheer force, while also trying to stab at their faces with spears.
Here you see the attackers making a suicidal and completely uncoordinated charge, while the defenders form a totally crappy attempt at a shield wall. There is nobody behind the frontline guys to brace them against the impact, and it happens exactly what you would expect to happen. They fall over.

But almost any other time, you get all these armies lined up as a single force, and as soon as they get close to each other they completely break formation with everyone individually throwing himself into a sea of whirling blades. Do you sometimes watch such battle scenes and wonder "how can they keep fighting without getting stabbed from behind, and who has the discipline to run into that blender?" Well, the answer is they can't and nobody does. It looks completely suicidal because it is.

Spoiler for Hiden:

Here we have a pretty good demonstration how it is supposed to look like, from a movie that doesn't even have the slightest pretense of being anything resembling realistic. But in an actual phalanx battle, this is how they would stay throughout the whole fighting.
There is a nice moment in The Hobbit 3 where the dwarves put up a really cool looking shield wall that is extremely well armored. And then those silly elves just jump over them into the orc horde and the dwarves say screw it and also abandon the whole maneuver. It is like the director saying "Hey, I know there are many of you who would like to see a somewhat realistic fight scene! Well, screw you, I won't give you any!"   

And you never ever run horses straight into this. Because you and your horse will 100% be dead. How it works is that the cavalry gets as close as they can without getting into reach of the spears, then throw their own spears, and all riders simultaneously turning their horses around and get away. Then they get new spears and do it again, over and over, each time maybe taking out a few defenders with a lucky throw that didn't get blocked by a shield. A bit more advanced is to do the same thing with lances that are longer than the spears of the defender. Then you don't have to throw your spear at the enemy (who could very well throw it back at you), but can use your speed and weight to have your lance make a very heavy impact on the shields. But you still all have to make the last minute turn before being impaled. Even in the late middle ages, when knights were very well armored, the horse was still vulnerable to some degree. The impact of a spear from a charging horse is pretty heavy, so you have to have a second and third row behind the first line of defenders, just to keep the ones in the front from being pushed over.
However, the defenders will still have some few small casualties and when you keep it up long enough (and actually can keep it up that long), eventually some individual defenders might think that the next charge might get them stabbed with a spear and try to run away. And then they are usually all very dead. A running man on foot has very little chance against a rider coming from behind and that's when you get total slaughters. If you know you are not going to win, its best to move backwards slowly and in order to leave the battlefield. With some luck your enemies will not pursue you. But if you turn and run, you're probably dead.

The Battle of Hastings took all with the Normans charging the defenders shield wall about a dozen times with no real effect. The Normans just couldn't get their enemies scared enough to run away in chaos, so instead they pretended to flee, so that their enemies charged forward in chaos. Then the riders simply turned around and all the defenders who were spread out over half the battlefield collectively thought "Oh Shit!" And that was the end of the Anglo-Saxon kingdoms.

[Justan Henner]

I agree with most of the above, but I think there's a couple things to consider.
While a fight scene may not realistically happen, as a writer, I feel like you have to think about what you gain from adding it anyways. For example, if you're trying to show a character's awesome warrior skills, then you need that scene. Or, if you're trying to show that a character kills first and asks questions later, a fight scene is vital.
While I love for a book to be realistic, if everything in a story was, where would the fun be? Also, if a writer isn't good at making the book exciting while keeping it completely real, then they should probably back up a bit.
Also keep in mind that while both sides have multiple goals, from my understanding, when you're in a fight things get crazy. Sometimes those goals fade into the background, and all you have left is the person standing in your way or preventing you from getting somewhere. For example, if you're a guard, when asked, your goal is not to kill the intruder. But when confronted by the reality of the situation, and the other guy may or may not be trying to kill you, the fear factor comes into play.
Of course, I've never actually been a guard or in combat, so maybe I'm wrong. But to put this in a modern-day scenario, if a guy comes into my house, most likely he's just a thief and doesn't mean any harm. Or, he could be a psychopath. I could try to fight him off until the police got there, or just let him do whatever and stay out of his way, but if he's crazy, I'm dead. So I would like to say I would kill him.
But let's say I don't. If I try to fight with him until the police gets here. What if he followed my above train of thought? He thinks that I'm trying to kill him because I think he's crazy, and so, in self-defense, he kills me.
The issue is uncertainty of the opponent. If you don't want a fight scene where they fight until the death, you have to make your intentions clear. Of course, depending on the character, the intentions that the opponent may be trying to show may not be understood.
In my opinion, a healthy mix of both types of fight scenes may be the better route. But I'm also not an actual writer, so take it for what it's worth to you.

Sorry I'm a few days behind this thread, so I haven't read the responses to this before I'm posting, which I really don't like to do, but I am going to anyway, because I really liked this post.

I loved Yora's points, but we shouldn't forget that a crucial element in this is how disciplined, and how well trained the character is. Thinking of the guard that only needs to defend, or the bandit that only needs to distract and get away, these are very true, and should be applied to the intelligent/experienced guard or bandit, but for the rest, just because it's logical, doesn't mean it's the response the inexperienced, or the 'character caught unaware' will have.

Anyway, back to reading the rest. I'm hoping the point hasn't already been made, but I'm assuming Yora has since he's a pro at this.

EDIT: It was the very next post, brought up by Raptori.  Also, I want to add that I clicked this...

Here you see the attackers making a suicidal and completely uncoordinated charge, while the defenders form a totally crappy attempt at a shield wall. There is nobody behind the frontline guys to brace them against the impact, and it happens exactly what you would expect to happen. They fall over.

expecting a bloody fight scene, and was completely surprised when I clicked over to an ad about baby ducks. Know your audience youtube.

[Yora]

I loved Yora's points, but we shouldn't forget that a crucial element in this is how disciplined, and how well trained the character is. Thinking of the guard that only needs to defend, or the bandit that only needs to distract and get away, these are very true, and should be applied to the intelligent/experienced guard or bandit, but for the rest, just because it's logical, doesn't mean it's the response the inexperienced, or the 'character caught unaware' will have.

Yes, things rarely turn out as planned. But when that happens, it should happen to inexperienced warriors who are then getting appropriately beaten. Or when a supposedly great warrior does it, he should not win anyway. The ones who fight terribly should be the ones who lose. Showing in detail how a character fights terribly and still wins without any trouble is something I really hate to see in fiction. Admitedly something that most people won't notice, because almost all fight scenes are badly scripted in that way.

[M.S OLNEY]

1. Pacing. If it's not exciting then what's the point?
2. Too much description breaks up the flow. In my mind a fight scene should flow fluidly and make a readers heart rate pick up.
3. Bernard Cornwell. (not a fantasy writer, but no one beat his battle scenes).

[Elfy]

As Yora said in a real fight things rarely turn out as planned, unless it's a WWE fight, then things turn out exactly as planned.