Walk-ons can be defined in the simplest of ways:

1. Name (this is optional, but the one time you don't have one will be the time the player's ask for it)
2. Purpose (what are they supposed to do in their scene)
3. Motivation - something they want or are dealing with that might have nothing to do with 1 and 2 - this is the sort of thing that will inspire a player to interacted with a walk-on to the point where they become more important.