Shot/ Reverse Shot

This type of shot is used to show that a character is looking at another character or object. Ussually the shot is close to the other characters face which eyes are pointing off-screen but because the characters are facing in the opposite direction, the audience will establish that they are making eye contact and therefore communicating with eachother. The 180 degree rule is also used while making this type of shot. When using this type of shot, the director must be very carefull that everything in the scene was exactly the same as it was when the next shot is taken from the other characters perspective. For example, if a character had their hand raised or was scratching their head, then when the shot switches round, that same character must have their hand and body in the same position. Many movies have made mistakes with this and props not being in the right place. Pretty much every film sequence and movie uses a shot/ reverse shot, where there is a conversation happening or communication between 2 or more characters.
http://www.youtube.com/watch?v=QLkUHZ1qips&feature=fvw

This movie shows the basic shot/ reverse shot. Everytime the other character is about to speak, the camera moves to look at them. This enables the audience to look at facial expressions and body language within that character. It is also very effective, because it happens quickly without the audience realising, whereas if the camera kept on moving and didnt cut, then it would look very unproffessional and cheap, and also very annoying. 

A very famous film called "the good, the bad and the ugly" very cleverly used a 3 way shot/ reverse shot during the final duel. http://www.youtube.com/watch?v=Y6x401CGAl8

The main part of the scene starts at 2.15 and from this point there is no dialogue until the enemy is shot. From 4.14 the director constantly uses shot/ reverse shots to build up tension because the audience knows that someone is going to die very soon. For example, at 4.52 it shows Lee Van Cleef looking to the right, and another shot at 4.53 it shows Eastwood looking to the left. But us as the audience autimatically assume that they are looking at eachother from the positions we saw them standing in earlier in the scene. This constantly happens with a few cut in shots of their hands near their pistols to build up the tension even further until Lee Van Cleef gets shot.