According to the LA Daily News, Hall has said that the character he is playing is so secret, he's forbidden to release any information about what he will actually be doing in the film.
The actor stated, "I signed a confidentiality agreement, and I can't say which part I'm playing because it affects the story. I can't give away the suspense, it's a $200 million surprise, and I don't want to be the guy to ruin it. I'm excited about it. Christian Bale is back as Batman. Heath Ledger is joining the cast as the Joker. Of course, Morgan Freeman and Michael Caine return, and they're great."
Hall was in the middle of shooting his sixth season of the USA series The Dead Zone when he got the call. Shooting had to be stalled for a day so that Hall could make the shoot. "My crew agreed to take a three-day weekend so I could fly Friday to go to London. It was just a thrill. I felt like a rock star leaving the set, flying to London, going to shoot on a Saturday, you know? My crew really gave me a gift because they had to all vote union-wise to agree to not take a day's pay."
The Dark Knight opens on June 18th, 2008.