The Main Character's preferred method of general problem solving
By temperament, Main Characters (like each of us) have a preferential method of approaching problems. Some would rather adapt their environment to themselves, others would rather adapt themselves to their environment. There is nothing intrinsically right or wrong with either approach, yet it does affect how one will respond to problems. Choosing "Do-er" or "Be-er" does not prevent a Main Character from using either approach, but merely defines the way he is likely to first approach a problem, using the other method only if the first one fails.