I got this question recently:
“What do you think of writing a screenplay ‘for’ specific actors? I don’t mean actually planning, or even hoping, to cast those actors–only to help visualize the scenes and vary the dialogue as you write. I’ve just finished the first draft of my first screenplay, and it came naturally to me to do it this way. It ‘s also great fun!”
I think this is an excellent idea and I know a lot of writers do this. Really anything you can do to try and breath life into your characters is probably worth trying. So if this helps you give your characters a more distinct voice than by all means do it.
Also check out these post as they answer similar questions: