We thought it might be helpful to include an example from a film about a real person. In this case, as a writer, you might have access to audiovisual examples of how the real person actually spoke and use them as a roadmap. 

Here, Alexander and Karaszewski developed their script with Eddie Murphy’s involvement. Even if you’re writing a screenplay on spec, having a specific actor and personality in mind can help to mold the way a character speaks on the page. You can write with their voice in your head.

Also, note how Jerry takes himself very seriously and Rudy has unflappable confidence in himself. This influences the passionate salesman vs. skeptical buyer way in which they speak to each other.