We had a Doula, and she recommended that we ask a bunch of questions on our initial visit. It wasn't a separate interview. We knew we'd move on to the next on the list, should we not like some of the answers (the c section one was the primary issue).

My wife interviewed several OB practices in the Chicago area before settling on one for delivering both of our children. In her group of friends, it was common. There was a small charge at each as an initial consultation.