In the theatre you can change things ever so slightly; it's an organic thing. Whereas in film you only have that chance on the day, and you have no control over it at all.
I started off as a theatre designer, and by some extraordinary circumstance I saw something in Stratford-upon-Avon, and realized that that's the kind of design I want, but also that that's the kind of designer I'll never be.
In contrast, the control you have in a theatre is very attractive to me.