Knowing the excerises and a few muscle names is olny enough to make you dangerous as with 95% of the weekend warriors that take a few classes and called themselves personal trainers .
At the very least you should be able to rattle off the name of every skeletal muscle with it's origin - insertion and direction of pull ... next would be a complete understanding of the three energy pathways the body uses ( CP , anaerobic A-lactate and aerobic ) now we're being very general here because those subjcts get very indepth ..
Moving along ... you come to exercise perscription and structure once you know the intended goals of your client ..
Then comes the learning what your own personality is and how you convay your message to others .. just because you know something does not mean you know how to teach it to someone else .
Diet and nutrition is huge so you best understand calories , food groups , energy pathways etc etc etc before giving anyone advise on eating . Where i live you have to be a certified nutrionalist inorder to persribe a diet to anyone or you can be held liable .
Personal training is not just about lifting weigths , you have to have an understanding of most sports as your going to get one client come to you and say " help me train for basketball " in which case your going to put them on a pyrometric outline . The next client may be a distance runner ... then next a powerlifter looking for more power , the next maybe a bodybuilder , the next a golfer , the next a 300 lb mom looking to lose weight , the next an average joe blow wanting to buff up for his high school reunion ..
As you can see ... personal training is more then hold someones hand in the gym while staring at yourself in the mirror and watching others admire you becasue your a " personal trainer "
If you don't know what your doing you can really fuck someone up ..
I have a phys-ed degree ( university ) plus i'm AFLCA certified in resistance training and have about 20 or 30 specialty cources ontop of that and i don't work in the industry fulltime ... I still write programs part time and correct those who have been steared in the wrong direction by weekend warriors .
If your going to do it .. do it right or you'll command zero respect within the industry once you start , be prepared to answer every question posed to you without having to go research the answer or you'll look like a fool