Let's start with the simple task of being heard, whether it's from the stage or in a film. Speaking clearly and loudly enough for the audience to understand what's been said is frequently a problem for many actors. When I watch their performances, I often wonder where they studied, or if they studied; turns out they did study...somewhere. Since most of us see many more films and watch TV more than we go to the theatre, I'll address myself to those mediums even though the task applies to and can be corrected in any medium.
It's not a matter of enunciation or "speaking up," it's a matter of understanding the most elementary aspect of the technique that any of the teachers I've referred to insisted upon; an aspect that is too often overlooked by people who're teaching today. Playing an objective (or action) and how ("intention," Meisner called it) to achieve that objective is the key to making oneself understood. Have you noticed that when you watch a scene (film or TV) you can hear every word from one of the actors, and not hear sentences or parts of sentences from the actor performing with him? If the same sound equipment is available for both actors, why is that? The only reason the actor we can understand knows how to respond to his partner is because he knows his partner's lines, not because he understood him. When an actor has a specific objective and knows how he intends to achieve it (moment to moment), he makes himself clear to his partner and thereby the audience. When that kind of involvement in the character's specific objectives is achieved, not only does the audience hear the actor, it also has the opportunity to become involved in the character's experience.
An actor who consistently keeps his objectives internalized; that is, he knows what he wants, but he doesn't seek to convey it to his partner, is Viggo Mortensen. This isn't a matter of talent, it's a matter of technique. Mr. Mortensen is a talented actor, but it's very difficult to hear him, therefore, it's difficult to understand him or what his character wants. I recently watched Edward Norton in Fight Club (I haven't seen enough of his work to know if this is consistent) and he, instead, begins what seems like an objective, and then, somewhere along his speech, parts of his sentences get lost. This occurred only because he hadn't chosen a clear, specific objective. If you watch the film (again?), notice that there's a little scene in which he tells Helena Bonham Carter's character to "...get out - leave..." (or words to that effect), and he made himself very clear. This was because he had a very specific objective and he knew how he'd let her know that he wanted her to "get out."
Of course another reason for playing the objective and the "how" has to do with achieving an authentic emotion rather than showing us the character's emotion or deciding how we want to sound or look, but I wanted to begin with being heard, because it's my pet peeve regarding the professional work that I see, and it tells me immediately that the actor is working incorrectly.
I pointed out the work of two actors here and I'll continue to point out work that is either correct or incorrect in the work of professional actors because their work is familiar and accessible to all of us, and from which we can all benefit and learn.