Is it illegal to watch another movie in the theater after you wathed one already?:
If you get caught, then the theater manager has the discretion to remove you from the theater and/or ban you from ever coming back.  This is dishonest-- it is stealing.  If you paid to see one film, and then sneak in to see a second film, then you have cheated the theater out of ticket sales.  You also may be affecting the box office totals for the film you saw but didn't pay to see.

People do this all the time and get away with it, but it is still not right.