Several differences come to mind right away...
...starting with the old cliche that Theatre is LIVE. It's happening right there in front of you, so there is an immediacy and visceral connection to it that you can't get from something filmed or prerecorded.
Theatre is an actor's and writer's medium.
Film is a director's medium.
What I mean is that with a stage play we seem to regard the writers as the creative "Gods" of the piece. The are the source from whom all other creative elements flow, including the directorial vision and the intent and shape of the characters in the piece.
But once the lights go down, the performers are out there alone doing their thing. Yes, they've been rehearsed and choreographed to present the piece a certain way, but at that point the show rests firmly on the shoulders of the people running the actual play (actors, musicians, a musical director, stage manager, technicians and other crew).
The director's job is completely over for that performance, once the lights go down. There isn't a damn thing that can be done to shape anything or fix anything from that position.
Film, on the other hand, is a director's medium. He (or she!) calls the shots (literally) before, during and AFTER the rest of the artistic elements have been created. He can decide in the editing room which "takes" he wants to use, and shape a performance in any number of potential ways. He can remove scenes, shots, words, moments that can completely change the intent and the meaning of an actor or writer. He can tighten or lengthen pacing and change POVs. In other words, the director is "god" in this medium.
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22