On some of the 7 night (or longer) cruises I have been on, they have had "behind the scenes" with the theater company. You meet the member of the cast and they give you info about the theater. We did this a number of years ago so maybe things have changed (I'm sure someone will update my info.) But we didn't go behind the curtains of the stage. It was just info about the theater. While it was very interesting, it wasn't what I would call a behind the scenes tour. (Don't know if they take you back there now or if there is some insurance liability in taking guests there.)
The other possibility is getting a "Magical Moment" from a cast member. Talk to Guest Services, the Cruise Staff, or if you meet any of the Theater Cast about your community theater and that you would love a more in depth tour. I understand the Cast Members might be able to create these Magical Moments for you (although we have never been lucky enough to get one!)