Giving a Minor Character Life:
What are some simple tricks to add instant personality to a minor character? Here are four techniques pros use to add a special touch to a character who's there to serve a function and not much else. Professionals use these tricks when they need something quick to bring someone to life. Major characters can also benefit occasionally from these tricks of the trade.
Establish "off-screen" activities. A character may enter a room very upset, looking for club soda to clean up a pizza spot on his tie from lunch. Or perhaps she's in a terrible hurry because she has an appointment to meet a landlord regarding a new apartment... and she really needs this apartment. This kind of business is quick to add, especially when introducing a character, and seems to be most effective when it doesn't have anything to do with the story at all. It's like these minor characters are in a story of their own and are not just in ours to serve a purpose.
Using props is a tried and true method of introducing a minor character. It's most effective when we see the prop before we meet the character. Perhaps we see their beat-up car or look over their diplomas on their doctor's office wall just before they stumble in drunk. Whether you're introducing them, or adding instant character, establishing a past through props is a quick and effective way to pull this off.
Give him a cold. It's simple and works instantly. We see this a lot, for some reason, with detectives. Usually, pros use this one to bring someone down to a more human level than either the hero or villain. It's the guy or girl who the audience is going to underestimate, the detective who figures it all out or the ally who comes to the rescue at the last second.
Go against the grain with a single characteristic. Figure out what purpose this minor character serves and give her an eccentricity that the reader would never expect. For example, a hit man with Coke-bottle thick glasses or a waitress who can't add two numbers together, or a desk clerk who has an accent so thick that no one can understand him. Look at what they do and figure out how to add one eccentricity to make her interesting. Notice we said "one." This technique seems to work best when the character is otherwise normal except for this one idiosyncrasy.