One factor to consider is the actor's life experience. You need to know this to give you an idea of what your actor has to draw from in telling your story. This is something to explore when discussing your script with actors. Another temptation is to choose actors whose essence is the essence of the character, i.e., choosing a cop to play a cop, or a trouble-maker to play a delinquent. There is some merit to this, but you need to be careful.
Most actors have difficulty just being themselves.
If you are considering such an actor, be sure to test for range and ability to take direction with added scrutiny. Since film is a technical process, you should also examine the actor's film and TV credits. Actors with mostly theatre credits present another challenge. These actors are naturally larger in their performance because they are constantly badgered to project to the back row of the theatre. In addition, they also have a tendency to act with their hands and arms.