It will be immensely helpful to learn about directing. I am an actor first, director second - with not a lot of film directing under my belt. But here's my take, from an actor's POV. Most film directors I've worked are not very good at getting the best performances out of their actors. Why? Because they don't know how to talk to actors in their language. Or they don't talk to them at all. They expect the actor to come to the set, lines down, character developed and ready to go, with minimal rehearsal. Sure, I can do that. But it won't necessarily be my best performance. A director needs to know what's possible from actors, as well. Actors can develop superb nuances within rehearsal or with the right discussion or coaching that can take a character from here to THERE, and knock it out of the park. The best way to learn actor-speak is to get onboard a professional theatre production (have an actor you know who's friendly with an artistic director or a director scheduled to direct a show and have them get you in the back door to observe as many rehearsals as possible). Or, if you can, get in to watch a Master Class in action. (Plenty in NYC or LA, London, not sure elsewhere). You want to see good actors working with a good director/teacher in order to see what's possible from actors. I took master classes from Uta Hagen and watching her critique other actors was amazing, nevermind the lessons I learned from her personally. Like a good direcctor (film or theatre), she could give such amazing notes on a scene - when I'd see the scene performed again, incorporating her notes, the difference was pretty incredible. So watching a class like this would really be great. I know some film directors may not be fans of theatrical acting - that's fine - stage actors sometimes have a hard time toning it down for film, but the lingo actors understand to develop complex characters - is pretty universal. I would bet there are some good DVDs out there.
Actor's Studio, or similar kind of venue, where you can watch actor's at work.