IGN: Robin Williams has been discussing his desire to appear in a Batman movie, stating that he'd play anything from The Riddler to an Arkham Asylum inmate in Batman 3.
Speaking to Empire Magazine about his work with director Christopher Nolan on Insomnia, Williams says "I would work with Chris again in a second, playing anyone in anything. I'd play The Riddler in the next Batman, although it would be hard to top Heath [Ledger] as the villain, and I'm a little hairy for tights."
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I think he would be an interesting choice. He plays a really good psycho guy, so it could work. But I think he might be too old for Nolan to consider him. He might want to keep the villains younger too.