My upcoming book finaled in a contest once, and part of the last step was to suggest actors who might play the roles in the story. This was difficult for me because I don't know the names of any actors under the age of forty. In fact, I don't know the names of many actors still living. Luckily my children were able to help, and I came up with a girl reminiscent of young Elizabeth, Ron Howard's daughter. (You see, I have to put them into a generation I can identify with!)
It's a helpful device, and I do it a lot in my head to help me imagine how a character would react in a given situation: what his face would look like, how he would sound, and what he might say. It worked with my current villain, who for some time didn't want to show me his personality. Finding him was easier once I attached a real person to my imagined bad guy. Even if the real one would never poison his way up the ladder of success, I've seen him in many moods: angry, sly, ebullient, and conciliatory. It's just one step more to picture him deciding, like my old buddy Macbeth, to take "the nearest way" and get what he wants.