What in your opinion is the biggest mistake people make while working out:Attempting to follow a program that a professional bodybuilder does prior to a competition. I think it is important for people to consider their own individual goals and situations when planning their health/fitness program. If you are not making a living off of your physique(endorsements & winnings) then it is not realistic to model after someone who is solely making a living from theirs. Take into consideration your lifestyle and make exercise & a healthy diet an enjoyable part of life.
Do you think a personal trainer should be also a good salesman:No, I don't think it is necessary. Someone talking to a trainer already has an idea of the benefits of a healthy lifestyle(nutrition & activity) so it then becomes a matter of compatibility. If there is rapport between the trainer and prospective client, fitness has already sold itself. I believe it is mostly a matter of personalities. If you both hit it off from the start and the prospect has been pre-screened there is not much need for selling at that point.
Describe a particularly challenging client you have trained:I worked with a wonderful lady for over four years who was dealing with end stage Parkinson's disease. Because of the nature of the illness I never knew what to expect when I showed up to work with her. Some days she was able to perform every task that I asked of her. Other days she had so little control that I basically kept her company until the tremors subsided. I learned a great deal from working with her. She was a great person.