It might be a bit hard for me to 'be' Old Man Willow in a film, it would all be computer generated, but I think being him in a stage production would be amazing. I would get to eat Merry and Pippin!
If I couldn't be Old Man Willow I might settle for being one of the hobbits. Frodo might be kind of cool, getting to complain and whine all the way to Mordor, but Pippin with all his naivete, would be more fun.
As a side note, I played the role of Galadriel in a small LOTR play when I was ten. I had no idea who or what she was, not having read the books then, but I had a fine time swishing around in a cloak.
