Poser

A person that believes heavily in cliques or social status, therefore acting differently around other people just to fit in. Posers usually end up having little or no friends in the end, because of these factors.

When people are not around:

Friend: Don't you think Star Wars is better than Titanic?
Poser: Hell yeah.

When people are around:
Friend: Don't you think Star Wars is better than Titanic?
Poser: Dude you're really gay.