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Think about your character's physical posture and habits, not just their lines. Pay attention to your reactions to other characters' lines, not just when it is "your turn. Think about the point of emphasis in each line or paragraph. Be consistent on stage or set to making shooting a breeze. Take improvisation classes to improve on-stage reactions and acting.
Human beings are enormously complex, and much of our communication comes from body language, not just words. So what is your character's posture like? Do they command the room or slouch in the corners? Do they move around or stay very still? Do they get really engaged when talking, or remain reserved and aloof?  You don't need to come up with unique tics and traits for every character, as this will be overkill for many parts. But you do need to think about how they stand, sit, and speak. Basic posture is easy to study if you're unsure -- walk into any restaurant or public setting and just people watch Separate yourself from the character. You might not use your hands when talking, but an older Italian character couldn't stop using them. " Great actors remain in character the entire scene, feeding off their co-actors to keep the scene alive even when they're not talking. Knowing the other actor's lines and being ready to respond to them makes the entire performance better, and helps you stay in character.  Real people (not characters being acted) stay in the moment. Don't think about your next scene or a flubbed line from before -- clue right into the present moment. Watch Charlie Day in the show It's Always Sunny in Philadelphia for a funny example of acting reactions. Even when he's in the background, he keeps up the nervous, unpredictable energy that makes his character (and the actor) so popular. Where you stress words matters as much as how fast you say them. Take the simple line, "I love you." You can stress all three words and get three different sounding sentences -- "I love you." (about "me"); "I love you" (about discovering love); or "I love you," (about them). You, as an actor, need to decide which of these moments is worth the most attention.  If in doubt, talk to the director. They may have a vision for the line or the character's arc that you need to mesh with. Emphasis is also important in monologues and paragraphs. In a long speech there is almost always a moment where the mood, subject, or idea evolves or switches. Find this point and make it a strong transition for your character. "Blocking" is where an actor stands, where they move, and when they do it. For film sets, this is usually meticulously mapped out to aid cameras, lights, and sound crews. It is essential that you stick to this blocking, otherwise you can ruin the entire shot, so don't try to improvise something new on the spot. Even theatrical actors need to stick to blocking consistently, as your movements dictate other actors' movements and some stage effects. Good actors rehearse and find the role before shooting. That way they can show up and consistently hit marks and lines instead of trying to figure it out on the fly. An improv class teaches you the art of acting in the present. To be good at improv, the character will always appear naturally, because it is happening in real time and not dictated by a script. This will help build the essential skill of reacting to the events in real time and not reading off a page. It also raises your on-stage awareness of other lines, actors, and props.  Many acting classes offer improv as a warm-up or a unit, so you can often get a little improv practice in with a "classic" acting class. When improvising, focus on building up the scene. Basically, this means always agreeing to your other actors, then adding a twist. To practice, work on responding to each line with a "Yes! and also..."