This interview touches on season three in general. I'd like to see who else was on this list:
You guys have had a lot of really fun, insane cameos this season, but I think [famous actor in the finale] might take the cake. Was that on the page or did you just slot in ”cool celebrity” and then look to see who was available?
First of all, he's amazing. Every time I see the episode, I think he's so funny and weird and did everything right. I'm sure he was on a short list that we had.
(This is one of those times where you really should avoid spoilers for everything about that episode to fully enjoy the surreality. A+)