You can have a look at other actors' performances, read books, and study behavior at home. However, plenty of the process of learning to act happens by watching your students. It can be hard to understand how a given concept or an ability applies to you, but much easier to see it in someone else whose work and challenges you're deeply familiar with through class.

Also, acting represents life, and life is not lived alone. In the same way that we need to interact with other people in order to have a full range of human experiences, you need to work with other actors at various levels of experience and in a variety of circumstances in order to truly learn the craft of acting.