1. Actors (at least those who use the "method" technique) learn to "become" the character they play on stage. Good writers "become" their protagonist when they are writing. They put themselves in the head of their characters so that everything they write feels authentic and natural.
2. Actors must learn to stretch their personal boundaries. They cannot let their inhibitions or the judgment of others stand in the way of their art. Writers need to do the same thing. They need to write what comes to them regardless of what others may think. Every time they put pen to paper, they need to think about how they can take what they are doing even father and go even deeper.
3. Actors must pour their soul into their work. Every night, no matter how many times they've performed the same part, actors must play it with the same freshness and energy they had on opening night. Fiction writers need to do the same when they pick up their pen. If writing becomes just another task or chore, it loses something important.