You can have all the qualities common to professionals (honesty, reliability, etc.) and all the qualities particular to the field of exercise (knowledge and experience in the field). That is often sufficient to be successful, but it might not be enough for the more advanced client or those who want to go to the “next level”. The really good trainers possess the quality of being able to be “in the moment"; this special quality will enable a trainer to take the client to the next level.
The next level? Someone exercising is willing to exercise at a certain level by themselves. That level is less than the absolute maximum they are capable of. A good fitness trainer who is in the moment will get the subject to go to the next level - somewhere above what they are willing to do and closer to what they are truly capable of. As one of my clients once told me, “I pay you to give me a workout I could not possibly get by myself”.