The film itself is nearing completion so Connolly's casting has come towards the end of production. Speaking on the subject, Jackson states: "We could not think of a more fitting actor to play Dain Ironfoot, the staunchest and toughest of Dwarves, than Billy Connolly, the Big Yin himself".
Connolly's no stranger to film appearances having starred in Mrs Brown and The Boondock Saints series, among others. According to Jackson, "with Billy stepping into this role, the cast of The Hobbit is now complete. We can't wait to see him on the battlefield!"
Neither can I. The first film, The Hobbit: An Unexpected Journey will be with us on December 14.
Source: Deadline

