Exercise your creative muscles all the time, either through classes or through other artistic avenues like painting or dancing or even gardening, staying active and being creative so that when opportunities do come up, you are in a position to take advantage of them.
You're trying to bring an emotional and psychological reality to a character that's on the page and make them three-dimensional.
As actors, you do have to be aware of your audience. You can't act in a vacuum, even if that audience isn't exactly present with you.
I think theater is a great training ground; I think it's a great opportunity to play.
If theatre paid as well as movies and TV, I'd do it all the time.
It's just a very different process from maybe working one or two days a week doing one or two scenes a day versus sustaining a performance over two hours or so eight times a week.