I pity directors who cast me in theatrical productions. They drill actors not only to know and understand their lines, but also the script's subtext and their characters' motivations. Directors impress upon actors the importance of characterization, rhythm, body language, blocking, vocal tone and more. And they insist, insist, insist that actors stay focused on the action and other characters on stage during a performance.
Yeah. Good luck with that last part.
If you want to see actors excited about theatre, get them talking about not motivation or vocal tone, but audience reaction. There's a reason people who are involved with community theatre laugh at the scene in Christopher Guest's movie Waiting for Guffman when the actors go ga-ga over the presence of an audience member -- it's because they've experienced it firsthand in their own dressing rooms and green rooms. As much as actors pay attention to what they're doing onstage, they're also paying attention to what lines got the biggest laughs.
Actors cannot help but notice the audience, which is natural. Theatre is the most collaborative of art forms, with numerous people being involved in its creation: the writer, director, designers, actors, crew and more. If a production were a cross-section of a planet, you'd see layer upon layer of points of view and perspectives so varied and numerous that the "core" of the planet -- the script -- ends up looking completely different than the planet itself -- the final production. Those perspectives are topped off by the audience reaction, which influences the performers, no matter how much they try not to be influenced: a cough from Row D may distract; the eerie silence of people paying attention during a soliloquy may strengthen an actor's self-confience; a peal of laughter from the house right area may affect the tempo; and a not-fully-shielded cell phone conversation from Section A may annoy and enrage. And all of it is discussed in dressing rooms far from an audience's eyes and ears.
Does this happen with other art forms? Nope. Ishmael and Captain Ahab won't be fazed if a copy of Moby-Dick is set aside for a couple of minutes so someone can pay the pizza delivery guy. The scene in Van Gogh's Starry Night doesn't change if someone brings a crying baby into the gallery. As for the ballet and the symphony, an audience is expected to clap only at the very end of a dance or musical piece. The prima ballerina or concertmaster usually must wait until they're finished performing before the audience provides its input.
Do I enjoy looking at paintings? Yes. Do I enjoy reading novels? Yes. Do I enjoy attending performances by the ballet and symphony orchestra? Yes and yes. But knowing that the actors are giving me something to react to, and they will be affected by my reaction while they're performing gives theatre an extra "spark" of entertainment for me. It's life responding to life.