Film is a young art form, a little more than 100 years old, with its origin and primary development from the U.S. and France. Theater originated centuries ago, during ancient religious ceremonies in Greece.
There are major differences in the technical requirements for movies and theater productions. And all these requirements affect the way performers should act.
A movie uses a camera to record an actor's performance. This requires acting skills favoring the right movements, angles and audience connection, based on how the actor will look when projected on a big screen. Subtle movements can be highly effective in this medium. In theater, the actor must connect with the audience through movements and voice projection on stage. Subtle actions won't be seen, for example, by observers in the more distant seats. Depending on the size and capacity of the theater, he must often exaggerate his movements while still communicating his character's actions and emotions.
The sound requirements for movie and theater are quite different. In movies, sound may be recorded live on set, or recorded later in a studio. Either way, the dialogue is usually just a part of the final sound mix, which may include music, sound effects, background noises and other audio elements required by the story. In theater, the actors speak live. Music can either be pre-recorded or performed live (or a mix of both).
Microphone use also varies. Unlike in movies, it is generally acceptable if people see the mic worn by a theater actor. The stage actor must speak loudly and enunciate her words very clearly--even if she wouldn't do so in a similar, real-life situation--so she can be heard by the entire audience. Movie actors generally speak in more lifelike tones.
Theater lighting is generally more dramatic, dynamic, sharp and high contrast. A spotlight typically focuses on specific actors during a performance.
In movies, the lighting depends on many variables, including the size of the shot, the continuity of the scene from one shot to the next, and the film's overall mood. For instance, films with a warm feel may feature sepia tones while movies intended to be cold often employ bluer tones with darker and longer shadows. The director's lighting treatment and shot compositions can also be manipulated during editing and other post-production work. The actors may also perform in front of blue or green screens, pretending actions are taking place on those screens. Those actions are added during post-production.
Preparing for a play requires many rehearsals and the memorization of all the lines. During the actual performance, there is no room for a mistake because the action is all live. In movies, rehearsals and line memorization may be similar, but the actual shot can be repeated as many times as the director wants, until the best shot is acquired. If an actor forgets his line, he can do the scene over again.