I can’t count how many times I’ve seen scripts where the writer has decided to “help” out the reader’s imagining by suggesting who can play the lead roles. In some cases it’s as innocuous as the character description in the text being “a Clint Eastwood type.” In others, it’s as obtrusive as creating an entire dramatis personae after the title page, suggesting the reader’s ideal cast. One of the strangest suggestions I ever saw in a script had a writer proposing that Michael Phelps would be an ideal choice to play an NFL quarterback. Suggesting actors is dicey enough, but it does the writer no favors to seriously propose non-actors for major roles, no matter what marketing viability they feel can come from their idea.
To put it gently, casting is something that is not the responsibility of the writer. That’s a complicated process that’s hashed out between the director, the producers and the studio, not to mention the casting director. The writer’s job is to create the parts, not to interpret them. You can certainly write your roles in a way that would make particular actors logical casting choices, and there’s nothing wrong with having your own personal “fantasy cast” for when you write. It’s when you include a note saying, “What about Gene Hackman for the part of the lawyer?” that you’ve crossed that invisible line. (For one thing, Gene has retired more or less, so making that suggestion only results in you appearing out of touch.)
Generally speaking, it’s best to just keep your mouth shut about casting unless specifically asked. And never, EVER, belittle the suggestions of the director or the producer. In the unlikely event they come in with the brilliant idea of casting a vapid “actress” from The Hills in your movie, say something like, “It’s funny it reads like that… I kept seeing her as a Katherine Heigl-type.”