Stage and film acting both require a well-stocked tool box of techniques, but they differ in several significant ways. Here are a few key areas.
Screen acting schools often stress to incoming students that screen and stage acting differ in a variety of ways -- and one of the first points may be that the way an actor experience known as character arc is different in the two media. Character arc refers to where a particular character is mentally and emotionally in the story at a given point. S/he typically changes in some way throughout the story, and on stage and screen, the audience sees this unfold chronologically, but films are shot out of sequence. This means you may shoot the last scene first or vice versa.
The actor must remember what information the character has at a given point and where s/he is on her "journey". Screen acting also requires repeating those moments for multiple takes, so the energy must be called upon each and every time. However, stage acting also requires this stamina, since you have to get through the entire story -- and in one go.
Character continuity is another element acting schools for film stress. In theatre, you develop a character over a period of time, during rehearsals. It can be easy to fall into trying to develop a character in the same way, over the span of film shoot. This can result in a very inconsistent character, however. While the character experiences changes through the story, the performance energy in a given scene and the overall character concept must have continuity for a believable, seamless performance.
Whether you're taking acting classes drama course for stage or screen, there are some similarities between the creative process for both. Developing a strong character always depends on extensive research of the script, the era or location of the story, the character's job and every other conceivable element that completes a strong mental picture of who this person is.
However, film acting can be a solo activity, as you typically don't get weeks of rehearsal with the cast and director. You must rehearse alone and sometimes film without any rehearsal at all. You may not even meet the other actors until this point. This requires the ability to be vulnerable and showing a strong relationship with the other character when the camera's rolling -- regardless of how well you know the actor.
Voice and Movement
One of the most significant adjustments made between stage and film acting is how you move and use your voice. On a stage, you must fill the space with your voice and gestures, so they're typically broader than in film. In film, the audience is much closer, so the focus is on tiny gestures and facial expressions, with particular emphasis on your eyes.
Not adjusting can make your performance seem garish or melodramatic. One difficult challenge for film acting in particular is learning to be more still. This is arguable beneficial in stage acting as well, but it's essential on screen, where every hand wave or head shake is visually amplified.
Film actors must also keep in mind physical continuity in a given scene -- which hand s/he is holding a glass with or which direction s/he turns at a given point, for instance. This allows the editor to splice together a smooth scene that doesn't look "cut up" jarring. The same goes for vocal choices like accents, pitch or even volume.