Worlds Collide: Valve And JJ Abrams Working On Movie

The moment either Chell or Gordon Freeman open their mouths one of their defining character traits will be effectively destroyed, and it'll just be downhill from there.

The whole movie in... First Person Perspective!

or even better;

The whole movie is a first person game and every viewer has a mouse and keyboard, every move from everyone is integrated/averaged together and this moves the character on the movie screen.