Every one wants to command, and no one wants to obey; and this is owing to the absence of that wonderful brahmacharya system of yore. First, learn to obey. The command will come by itself. Always first learn to be a servant, and then you will be fit to be a master.
I approach every part I'm asked to do and decide to do from exactly the same angle: who is this person, what does he want, how does he attempt to get it, and what happens to him when he doesn't get it, or if he does?