How do you train when you are not in class?
You observe. You take note. You live your life.
As an actor a large percentage of the resources you will need to create a character are within you. The things you have seen, heard, felt and experienced are all stored away waiting to be accessed and used as a starting point for, an enhancement to or an understanding of a character.
Part of your responsibility is to continually expand your library of resources. If you don’t do that your performances are in danger of becoming static, repetitive and formulaic.
As you move through your day, observe. Observe the person sitting across from you on the bus, next to you in their car waiting for the light to change, or at the table near you in the restaurant. Watch how they move, how they sit still, how they stand, how they eat. What energy is emanating from them? Are they lost in thought? Do they seem melancholy, joyous, contemplative? Observe how their bodies adjust based on what they are thinking. So much to observe!
May I suggest the obvious and say that this exercise needs to be done discreetly. As soon as someone recognizes that you are scrutinizing them, not only does their behavior become stilted but you come across as creepy. This exercise also needs to be done with respect. No judgement allowed. Just observe what you see for what it is. Each of us are perfect in our own way and perfection has many faces.
The same applies to things you hear, feel and experience. Make note of them all and store them away for later use. Pick select times during your day to do active observing. The majority of your time should be spent in the moment experiencing your life to the fullest. Everything you do will move you toward being a better actor!