Shot/Reverse shot:

This is a film technique where two characters usually having a conversation in a scene would be filmed separately. One character is shown looking (usually off screen), and then the other character is shown 'looking back'. The characters are exposed facing opposite directions and therefore the viewers unconciously assume they are looking at each other. http://www.youtube.com/watch?v=4gIdceyd1L8
In this example of F.R.I.E.N.D.S, you can clearly see that the 'shot/reverse shot' technique is being used. Character A in the grey jacket appears to be looking off screen, and is being filmed alone. Same applies for Character B as he is shown looking back. A and B are continually having their conversation and this is what makes the audience assume they are looking at one another.