You get a job acting in a feature film, you go to work every day, read your lines, do your best, and when it's done, you go on to the next project. After your work ends, shots are selected, the footage is edited, and sequences are rearranged. Special effects are added. Music is added. And then there are all those scenes that you weren't in that could have contained anything. By the time you attend the premiere months later, you don't even recognize the movie. It's happened to quite a few actors. In fact, some were totally in the dark about what the director's finished vision of the film was all along.
There were actors who didn't know they were making a horror film. Others that didn't know all there was to know about their own character. And at least one was surprised to find himself almost cut out of the film altogether, even though he was the main character! Read six stories about actors in the dark at Cracked. -via mental_floss
http://www.imdb.com/title/tt0080491/?ref_=fn_al_tt_1