Whatever situation your mirror character was in before just got more complicated. Will he respond the same as you to the situation now that he is motivated by friendship, career, time, opportunity, and conflict. Give him a life. The foregoing was general motivation - those things that are different about each of our lives, like career goals, that make us unique from each other. What about specific motivation - immediate problems, needs and desires.
Many characters walk sideways through a story never confronting anything.
They cast no shadow because they're not really there - they don't care about anything. Because Jerry doesn't care if he makes it or not. Jerry s father always thought Jerry a bum. They re about to foreclose on Jerry's house and kick him and his daughter out. Jerry loves comedy - Jerry is motivated and the audience begins to care.