What Is Good Preparation For An Actor?

Well that all depends on what you mean by preparation.  Preparation can mean simple things like turning up on time, doing what's required of you and doing it with focus, energy and commitment.  Not going out the night before you are needed can make you sharper and more focused.  Knowing your lines early won't hurt your work. Being able to offer suggestions or try riskier things (not physically riskier).

Other forms of preparation include making sure that you have a strong, supple voice and body, so that you are prepared to do the worker when the need arises, increasing the physical skills you have, so if you are required to ride horses or drive a car, you already know how to do those things.

Preparation simply indicates that one should be ready.  Preparation is all we can do in this profession, everything else exists in the moment.  Preparation is everything.