Both the director and the producer are very complex types of work, and the accumulation of experience is very important. If you haven't experienced the hard work on the set, your ability to execute on the spot will be problematic. Therefore, in some countries where the film industry is mature, after graduating from school, newcomers must first be apprentices in studios, starting with script writing, then going through the assistant director, executive director and other links. After mastering all the skills, they will eventually become a qualified director, then guide and supervise.
The truth contained in this passage can be best summarized as: the knowledge and principles obtained from books are simple, but to truly understand them, one must personally verify them.