Translation:The actor prepares himself for the theater.
I know it's a two year old question, but the responses given so far do not quite answer the question.
In prepari sin the person is doing the action. He's making himself prepared. In prepariĝi there's no reference to who is doing the preparation, and it might happen without anybody actually doing it.
- La aktoro preparis sin... = the actor prepared himself.
- La aktoro prepariĝis ... = the actor got prepared... the actor was made ready (by who?) ... the actor prepared...
I tried to answer this question in my blog and it turned into a three part series.
From my limited understanding, if I hear "la aktoro preparas por la teatro", I would feel compelled to ask "Kion li preparas?" (What does he prepare?)
English can do without a pronoun there, as the reflexive is implied. "He prepares" means "he gets ready". The "himself" seems to me a mere hint that the translation needs something there.
I think the Esperanto verb works differently, and needs an object.
Either, you prepare "you(rself)", you prepare someone (else), or you prepare something.