Tom: What did you do last weekend? Mona: I went to SEE that new movie that everyone is talking about. (I think that "watch" in this sentence would sound strange, but I cannot explain why.) Tom: What did you do last weekend? Mona: Nothing much. I just WATCHED a lot of TV. (I think that "saw" in this sentence would sound a little strange -- to me, at least.) When there's an accident on the freeway, other drivers often slow down to ____ what's going on. (I think that either infinitive would fit here.)

To watch is to look attentively. To see is much more general in the sense that we perceive by sight or have the power to perceive by sight... In a conversation about a movie, play, sporting event etc... Either statement is correct. The one you personally like will fit your personal expression the best.

There's not much difference. I think you can use either =)