While there are several differences, the major one is how each is produced:
Movies are shot over a number of weeks with a pre-determined budget. If a performance isn't living up to the director's expectations, scenes can be re-shot as many times as the director desires. When the movie fits the director's vision, it can be edited together and special effects added.
On the other hand, a play is rehearsed over a set number of days (or weeks), and then performed live, in front of an audience, in real time. Unlike movies, there are no "second chances" in live theater: if an actor flubs a line, or something happens on stage that shouldn't (for example, a prop falling onto the floor in the middle of an actor's line), the action can't stop and the scene performed again – the show most continue regardless of any mistakes.