Any good personal trainer when they meet a potential client for the first time needs to first explain what genetic predisposition the person has. This is called somatotyping which puts your body into a category that explains the lean muscle to fat distribution that you have genetically.
Endomorphs for example will tend to put on fat and muscle just as quickly so one needs to be very aware of the quality of the calories you eat if you are an endomorph. There are three categories but one will never find the perfect ectomorph or the perfect mesomorph, it is just a scientific measure.
The Ectomorph is the tall and thin person who is always a hard gainer and usually has a very low body-fat and a very fast metabolism. This type of person will definitely be able to gain more muscle but it will take a lot longer because of the muscle fiber allocation between red (fast twitch) and white (slow twitch) muscle fibers.
If we train correctly using the correct sets and reps as well as the correct movements then there is a stronger possibility of achieving success. For example the average mesomorph will be able to gain muscle faster than the other two somatotypes and it is a good idea to adapt your specific program to suit your somatotype.
Although putting on muscle for all three groups is going to mean doing basic compound movements, the intensity and the sets and reps can be changed specifically to deal directly with any somatotyping challenge in mind. For an example doing cardio for an ectomorph who wants to add muscle is a complete waste of time.
There are no true Endomorphs or Mesomorphs we are all a mixture of all three with the one being more dominant than the other two. When we see someone who is tall and thin they are obviously primarily an Ectomorph and if he wants to gain muscle then he needs to learn the fastest way to get strong.
Whereas when you are dealing with a man of medium height who is obviously a Mesomorph then he would be advised to eat as much as he wants and train as hard as he can for an hour a day. If a Mesomorph trains with weights every day he will pick up muscle definitely along with correct nutrition.
It boils down to two somatotypes that need extra care taken with their training program and their nutrition. The first is the tall and thin Ectomorph and the second is the Endomorph which is the overweight or over-fat man. The mesomorph picks up muscle fast but it needs to be on calorie restricted