The 180 degree rule is the most basic rule in filmmaking. There is a really good short primer on it here (thanks wikipedia!). I will attempt to explain it briefly for those who are unfamiliar with the rule, but mostly I want to talk about the fact that live action filmmakers break it occasionally, while in animated films (at least in my experience at Disney) it is considered a big taboo. On most of the animated films I’ve worked on it would never even be considered as a remote possibility, and I think that is too bad, because sometimes breaking the 180 rule can result in a better visual impact and can also enable you to get a great reaction shot that you couldn’t get otherwise.

Okay, first a simple explanation: if you have two characters talking, draw an imaginary line between them. Now the rule states that you need to keep the camera on one side of that line and never cross over to the other side.

You can put the camera anywhere you want as long as you don’t cross the line to the other side of the two characters. This way, no matter what shots you have, you can cut them together in any order and the green character will always stay on the right side of the frame and the blue character will always stay on the left.

If you break this rule and shoot one shot from the other side of the line, the characters will be flopped: the blue guy is now on the right and the green guy is on the left.

This can confuse the audience because, for example, if the characters look similar, they may start to get the two people mixed up. Or they may think that the characters switched places between cuts, or they may think it’s a time jump to a different location at a later time or something. It can cause unnecessary confusion in the audience’s mind, and we always want to avoid that.

The problem becomes even more apparent when you’re doing a scene where people are in action. For example, when a character is running, you want to consider the path they’re traveling along as the line that you don’t want to cross. Obviously, if you shoot from the other side the line, the character will look like he’s going the opposite direction.

If you start to cut these two different shots together you will create a lot of confusion: did the character turn around and start running back the other way? Or is it two characters running towards each other and they’re going to collide? 

That’s why you’ll notice that - especially in animated movies - a destination is always kept to one side of the screen or the other and the character is always traveling that way. A good example is in Disney’s “Bolt” - Bolt is traveling from New York to Los Angeles and he is always traveling towards screen left. All of us Americans grew up looking at maps of the US with NY on the right hand side and LA on the left so this inherently makes sense to us and it helps keep us clear that he’s always traveling towards his goal and not suddenly turning around and heading back towards the East Coast. (Read the rest of the awesome article here.)