If you're having a hard time coming up with a character, period, look at your story and find all the tangential people who could become involved. For example, if you're writing a fic in which Spike is playing poker, think of the types of people (or demons) he'd be playing with. Choose one to write about. On a similar note, what about the waitress bringing them their drinks? How does she react to the situation? You might never use this character in your story, but it's a great way to imagine how original characters could become involved.