Declining a Role After you Accepted It

What is the best, most professional way to talk with a director about possibly declining a role. Was offered a part as a Phantom in Rocky Horror, but got a callback for Dracula (Mina)at a theatre that is a bit of a step up from the other. Already accepted Phantom because it didn't sound like anything would come of the other. Rocky rehearsal starts tonight;my thought is to just be honest and up front about what is going on...any thoughts?
This is community theatre here, but I'd still like to be polite and professional.

I would also conceder the fact that you might not get the other role and be left with nothing. I hate declining roles, but doing it as professional as possible won't leave as much of a bad taste in the directors mouth.

Less is more, just be simple and upfront. And polite. Graceful.
Community Theatre directors are notorious for burning bridges left and right
don't go get ya'self blacklisted.

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