No, it can't be guaranteed, but if you get there early and look like you are having fun, it increases the chances for some disney magic.
My sister, brother and I were chosen, 3 adults, who would have thought?! We had a blast doing it. I was enjoying the Dapper Dans singing so much. Who else can sing "How much do you weigh?" as part of the birthday song and get a laugh from a 40 year old woman? The CM who does the picking was informed by the Dapper Dans that if he didn't pick us, he was making a mistake. The rest of the day was so fun!
Marie