The thing that makes me most successful as an intervener is lowering expectations. If people think I am a miracle worker then I can rarely live up to their expectations. If they think I am somebody who is no threat to them and who therefore has some license to ask a lot of dumb questions and if I acknowledge their belief about how messed up the situation is, and how difficult it is and how unlikely anything positive can possibly happen, then when something positive does happen it clearly wasn't my work because I am a dumb jerk. It's their work. They feel good about it. They take credit for it, they actually have some ownership of the end product, and they want to see it go forward and succeed. The combination of lowered expectations and dumb questions would seem to be my particular approach. Now other people might use expertise, but I rarely do.