If an actor has the habit of fitness exercise, it is certainly worth learning, but if it is for the role to gain muscle or lose weight quickly in a short time, these "demonstrations" may not be good demonstrations.
First of all: the actor is not exercising by himself, nor is he simply hiring a personal trainer, but a team is helping him achieve his goal. What the public sees is only the result. Only the actors themselves know how painful this process is. Of course, the cost of this process is also huge. Even if the coaching team doesn't want money, the average person can't afford to talk about the diet plan.
Secondly: even if you gain muscle, you should diet moderately for the visual effect on the screen. Hugh Jackman, who plays Wolverine, is like this. He plays Wolverine in 17, so he strictly controls his diet almost all the year round. In order to highlight the texture and clarity of muscles and burst blood vessels, he should control carbohydrates and dehydrate moderately, especially the latter is not good for his health.
Third: Sometimes I have to keep eating for the exercise of my role! Weight gain may just ignore calories, but weight loss and muscle gain should consider calories. For the growth of muscles, the body must be in a state of positive nitrogen balance in protein, which requires eating more protein in small meals, constantly eating protein's food, and eating simple and cooked meat with no taste at all. Although ingesting protein as required is harmless to the body, single boring food and eating less and more meals are great mental torture, and dehydration is even more unbearable.