Second, lines with clear rhythm. Tempo.
This is particularly important with verse plays (Shakespeare, Moliere), but applies to lyrical or poetic plays, too. Change a word in Equus, which is very poetic, and you mess with the character, with the meaning of the play, with the rhythm (and therefore magic) of the piece. Plays which weave a spell around the audience do so in large part because the writing permits it. Change the writing, and you may break the spell.
Remember, I said that studies have shown that when the spell is broken, it takes up to five minutes to get an audience connected back to the play? Changing the words can break the spell. Do it at your peril.
Literary devices like onomatopoeia. Assonance. Alliteration. Parallelism. Etc.
If a sentence has a staccato feel to it, that is probably intentional on the playwright’s part. It may reflect a character’s jitteriness, for example. A sentence with powerful, active words may be spoken by someone in anger, someone who expresses themselves very physically. Change a word, and you may limit your ability, as an actor, to express a character’s emotions, because you’ll be disconnected from what the playwright actually wrote.
For instance, let’s say you have the line, “He blasted into the room,” or perhaps, “He burst into the room.” Change the line to “He came into the room” or “He rushed into the room” or even “He pushed his way into the room”, and you’ve changed the feel of it. The plosive “B” in the first two matches the feel of a door swinging hard against the wall as it is thrown open.
Playwrights are just as prone as novelists and poets to spend days agonizing over the choice of a single word. I’ve seen actors change the way a line is worded because “it feels more comfortable.” Because “I think this is how he’d say it.” Because “this makes more sense to me.” Quite frankly, this makes me want to scream just a little bit. The actor has probably only worked with the script for a few weeks when he casually makes this decision. Forget the fact that the playwright lived with the play for months, maybe years. Forget that the playwright may have once written the line exactly as the actor is proposing, and changed it because he decided the word he replaced it with was a better choice.
Bottom Line: If you think you know better than the playwright, it probably means you haven’t studied the script enough.
See Part I here.