Film and stage are quite similar in that they both are looking for truthful behavior within imaginary circumstances, or to quote Stanislavski, "Actors must live privately in public" regardless of medium.
Unfortunately there is a lot of misunderstanding of both delivery mechanisms that leads to the belief that the two are very dissimilar.The difference lies in the technical demand of each and in the way in which audiences view the material. I am just adding here to many of the comments that have gone before.
On stage, the actor must draw focus; on camera the camera focuses on whatever the director wants to see.
On stage, the actor has the ability to repeat a performance night after night which usually deepens their per and therefore the emotional wallop of the play. On camera, the actor repeats a scene or a sequence over and over, which should, if the actor and the director are in sync, produce different and or better work.
On stage, the actor does a scene and goes off stage for only a brief moment of time and rarely leaves the immediate environs of the stage itself. Theatre usually takes about two hours to accomplish the drama. On camera, productions needs to stop and set up new lights and camera angles, or to move from place to place. The concentration needed by the stage actor is not as difficult as it is for the possible 8 to 10 hours on set required of on camera acting where the actor moves out of the set and into his or her trailer. For this reason, many film actors may be criticized as ‘too method’ or as hard to work with, or as unfriendly. In reality, they need to maintain the emotional commitment to the scenes they are being asked to shoot that day.
On stage, actors must find their light and maintain the staging agreed upon in rehearsal. However, minor changes in staging don’t affect the overall requirements of the production. On film, actors also need to be aware of hitting marks which is governed by lighting, and the general plan for shooting whether close ups, two shots or long shots.
On stage, the actor must give enough voice to be heard in the theatre in which he/she is working. Voices on stage must be energetic enough to draw focus and to excite the audience’s attention. On film, the actor must allow the microphone to magnify his or her voice. However, with the growing trend of using mics on stage, this difference is fading away.
On stage, the actor must realize what the dramatic structure of the scene is regarding its action beats (bits in Russian) and through his or her interactions, make sure they occur. On film, the shooting primarily takes care of these through editing and shooting sequences.
On stage, the actor has the ability to do all of his or her work within the brief time limit of the performance. Film is more like some theatre rehearsal techniques in which the play is rehearsed piecemeal before it is put together. Film is shot out of sequence and therefore, like stage, the actor must recognize the preceding action.
On stage, the actor’s physical movements are fairly unrestricted. On camera unrestricted movement is more likely in long shots and less so in close-ups. In both, efficiency of movement is valued. There is also the need for film actors to be a bit slower in moving, at least in close ups and two shots, because of the camera’s need to follow them. A great commercial director, Leslie Dektor, once said that actor’s should imagine that the camera is on a chain at some distance from them. With that image, the actor should understand that not only is he or she connected to the camera, but that sudden unplanned movement would whip the camera around like a ball on the end of a string, or like the “whip” ride at a carnival.
On stage, the actor’s text is usually meant to convey those things that aren’t seen or the feelings the character have. Film does most of that for the actor simply through pictures and music. There is a basic tenet of film that as much as possible the camera should tell the story without words.
On stage, actors can make character choices that are extreme because stage is not based in realism. Film is essentially a realistic medium and the actor’s need for deep personalization is more necessary (sorry to say) than stage. Of course, the stage choices that actors make are also delimited by the size of the theater. In Chicago, most of our theaters are between 40 and 150 seats, which leads to a more intimate style of acting. Perhaps that is why so many Chicago actors make the easy transition to camera which adds to our becoming a hub of film production.
On stage, the script is holy, while on camera, the script is often only a guide. This depends of the directors and writer involved, but as a rule of thumb, it is true.
On stage, actors and directors generally work to create an ensemble, wherein the small roles are as important as the lead roles and with the exception of Broadway, there are no stars. Film is often based on its leading actor’s drawing power, and frequently all other roles are seen as ancillary to the stars who bring in the money. This is of course a generalization, because as a casting director myself, I have worked with many directors who come from an ensemble based way of working i.e. the Coen brothers.
On stage, the audience is a participant in the actor’s performance whether they realize it or not. They also are almost constantly aware of other members of the audience and in some theatre configurations they see each other. On camera, the audience, after it has finished the popcorn, is in an altered state, in a way mesmerized by the size of the screen, the music, the color of the film. They enter into it and more or less become the characters. For this reason, actors are often told that less is more.
Why? Because the audience members are in many ways virtually inside the performance with the actors and too much emotion or sound or whatever can break the audience’s ability to create their own performance.