A healthy combination of results and charm will win them over. Once they trust you the relationship will flourish. An ineffective consultant has a big ego, is impatient, has a short temper and easily gives up. Consultants who are stubborn and think they are always right don't work either. Sometimes consultants don't listen to what an attorney has to say because they were hired by a firm to direct efforts, but an attorney's suggestions will help you get to know them better, how they think and make them feel good about participating in the process.
Cooperation is important.
Hope this stream of consciousness helps. One thing I hate and thankfully doesn't happen to me very often, is to have a consultant try to work around me or exclude me. Everybody wants to learn, but nobody wants to be taught. I recall that there is a technique called the five whys. Some people aren't going to be ready to hear you, no matter what.