If you volunteer for a part in the show, it is likely that you will get it. From what I've seen, a lot of people are shy and don't want to volunteer, so if someone steps up, they typically get to participate. In my case, being a 6'1'' guy with a beard, I've seen the show 4 times, been a part of the show 3 of those 4 times, and volunteered ZERO times. If you have a tall male in the room, they'll likely be asked to be a Suit of Armor. After all of the main parts are given out, they'll ask the room if there are any other guests that want to participate. They'll be given the role of various dishes and silverware for the Be Our Guest segment. I'm not CERTAIN about the picture.. Like I said, I'm almost always asked to be in the show, we get a picture, and then we leave, so I don't know if they let other guests stick around for a picture.
Overall, this is one of the most underrated attractions in the park. The show is a lot of fun and the special effects are off the charts. Disney did a great job with this one.