He was pretty much perfect for the role. I'm not saying he did anything absolutely spectacular, but he did absolutely everything required for the role of Gordon- of course the role wouldn't be as lively or memorable as The Joker (or even Batman/ Harvey Dent). Also, one must remember that that isn't really a role one would expect of Gary Oldman, who generally seems to play psychopathic bad guys (True Romance, Air Force One, Leon, Dracula, etc.). Plus his American accent is, as far as I can tell, pretty damn good (though I am English, so I could be mistaken; I've not heard any complaints, either this time or in other times he's used an American accent).
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Thanks to Badwolf for the great sig!