Most unexpected guest voice

I don't mean across least likely to do the show, but a voice you didn't recognize until you decided to look it up. For me it's Peter Serafinowicz as Scottjon Dan Steve and Adam Driver as Art the Artist. Now that I know it's Adam Driver I can't unhear it but definitely didn't realize it was him at first. And up until about 20 minutes ago I thought Scottjon was a show regular doing their best Hugh Grant impersonation.