I was selected for the USH version back in 2007. It was a fairly quiet Saturday in late October 2007 and I'd purchased a Front of the Line Pass. I walked up to my designated area (USH's Express Line entrance) and the cast member asked me if I wanted to volunteer to be in the show. I said "Heck yeah!" and the next thing I knew I was whisked backstage. The Host of the show kept giving me instructions on what to do, but he gave them to me so fast that I couldn't understand what he said and he was gone (to the main stage) to give the performance before I could even say "Wait!" Needless to say, I screwed up but I hammed it up to the best of my abilities and the audience really seemed to enjoy it. It was an absolute blast, and it was fun to see things from "behind the curtain"...especially the actor who played the "animatronic werewolf" chilling out backstage, smoking a cigarette (likely against the rules). As I walked out of the show at the end, they were selecting new volunteer/victims for the next show, using the people at the front of the FLP line once again.
The show in Orlando pales in comparison to the show that got axed for Transformers in Hollywood (haven't seen the "new" version of the show yet). They had me WAY (30+ feet) up in the air on a set with a TINY little catwalk platform, tied up and being tortured in one scene (before "magically" turning me into a skeleton), then they had me "attacking" another female volunteer with a GIANT fake cat's arm/claw to supplement rear-screen projection of a giant cat. For a third scene, they "slit" my arm open with a stage knife/fake blood, and there was one other scene they used me for that I don't remember. What a blast!
And as far as I can tell from having seen the UOR version several times, they aren't using plants.