Maybe you're someone who prefers a slave-driver or maybe you'd prefer a personal trainer like Gary, who believes in striking a good rapport with his clients. And to do that, having a good sense of humour certainly helps. Sometimes, he feels more like a confidante, a listener, a friend than a trainer. It's more like a friend-friend basis. While the trainer may have the necessary certification, it is also important to find someone who possesses enough experience to teach all the skills.
Experience depends on the number of hours of training per year.
The minimum requirement for an experienced trainer is 100 hours of coaching per month or 1,200 hours per year. He will be able to design alternative exercises for clients should they not work out. A trainer can claim to know the theory by heart, but he might not have experience to know what works best if he hasn't tried them himself.