How about a little real-world friction? At least two of the characters don't like each other. As in, "Wouldn't spend a minute of my life with you socially," but respect each other, work well together and can depend on each other. (Opposed to the much overdone "I don't like you and will jeopardize everyone's lives because I'm childish" garbage.)
Just occurred to me that Tony Stark and Steve Rogers are close to what I'm talking about. At least before the Civil War.
