Years ago I took a beginning stage acting class. The teacher one day asked us to play the part of someone looking for a five dollar bill. We looked around, put index fingers to chin in looks of questioning and sudden realization, jumped into action and looked behind the chair. Then the teacher said, "Okay. I've actually hidden a five dollar bill in the room. Find it." You've never seen a room full of grad students move so fast.
The lesson: acting is not imitating. It's doing. Actors want a role they can get inside: something that makes sense, holds drama, and lets them experience life from a new perspective. The last thing they really want is a screenwriter to act like a puppeteer. They don't want you to tell them what to do. They want you to create a compelling character through a series of actions. Then THEY want to interpret them.
Let's take the scene we talked about earlier, when Ratso Rizzo invites Joe Buck to stay with him. Do you think it's full of action lines telling us about the subtle details of their faces? Nope. Do you think it's full of dialogue where they hash it out? Nope. It's dead simple. Six lines or so. Then screenwriter Waldo Salt hands it off to Dustin Hoffman and Jon Voight. And he wins the Academy Award for screenwriting.
What he's handing off to these genius actors is, of course, some genius writing. These are great characters not because of how they're described, but because of the situations they're in. Rizzo is dead lonely. He's broke and starving. The only guy talking to him wants to kill him. He needs to convince him to stay with him, or he'll die alone. Joe Buck's more or less in the same situation, and he's finally got a place to live if he can push his pride down one more time.
Don't tell the actors what to imitate. Just give them juicy roles to explore for themselves. That's what makes the drama spring to life.
Screenwriting is just the first step in a long creative and collaborative process. Working with filmmakers quickly teaches you that EVERYONE, from the actors, to the directors, to the gaffer, is trying to tell the story using their own creativity. Screenwriters have a very special role at the beginning of this process. You're creating the space for these other professionals to express their own creativity.