I was a "Professional" Girl Scout for little over a year back in the early 90s. Most likely, the person you are going to interview with has a job similar to the one I had. I recruited adult volunteers and then trained them to be wonderful Girl Scout Leaders. Unless things have changed, the Field Director/Consultant (that's what we were, not sure if the title is still the same) is jumping for joy that a non-mommy is interested in being a leader.
Don't worry so much about the interview. I usually wanted to know why the person was interested in being a leader (to keep out the wierd-os), what kinds of experiences they've had in the past working with girls and what they like to do in their spare time (usually an indicatation of what they would do with the girls). I would also try to get an idea of how much time they could TRULY commit on a weekly basis. Some people under estimate the time comittment it takes, and then would end up dropping out or doing a half-assed job, which, was, of course really bad for the girls. Unless the Council is overflowing with adult volunteers (highly unlikely) you will probably get placed. Be honest about what your expectations are, what you are looking for from the experience and how much time you have to give. Expect to go through a back ground check (pretty standard issue for anyone working with children now a days--or it should be)
In addition, don't worry if you have never done anything like that before. The National GS office provides excellent leader training, so you will be able to take advantage of that.
One word of caution, FDs have quotas they have to meet for new troups started, number of girls served, etc. Some are more concerned about their numbers that the girls having a good experience with Girl Scouts. If your contact is like that, and I hope she is not, don't let her pressure you into taking the troup by yourself with the promise that "the mommies" will help out. You need a FIRM committment from someone to be your co-leader or the leader (if you would prefer to be the assistant). Let me tell you, it is hard to say no when you have 30 little girls bouncing off the walls in front of you excited about the posibility of being a girl scout!
Whew! Didn't mean to write a book! Let me know if you have questions. I know of at least 1 other former professional Girl Scout around GC that might be able to help you as well!
Good luck and let us know how it turns out.