I don't know how they pick them, but my son was the sheriff many years ago. I never laughed so hard in my life. He is a quiet but self confident kid (still is, 15 years later) and had such total self possession that it was funny. Never cracked a smile - total concentration. It was priceless. We didn't have a camera, and they took a Polaroid for us.
I can tell you what NOT to do to get picked. My brother, who hates crowd participation things, wasn't singing along during one of the songs, and they stopped the music. One of the pretty girls sat on his lap, twirled his hair, and got him to do the "toot toot" part of the song really loudly. We thought it was hysterical - he hated it but was a really good sport about it. All the way back to our car people kept recognizing him and congratulating him.
Good luck - hope he gets picked!