Dignam [SPOILER]
Why was Dignam inserted as a major character into the remake? (I find it unbelievable if the reason was to avoid the ending problem that "Infernal Affairs" had with China's censor.).
After Queenan had died, Costigan could have tried to seek Dignam's help (to restore Costigan's rank) and Sullivan could have as well (to use Dignam's knowledge of the rat).
Presumably, Dignam thought Sullivan murdered Costigan. As one who had believed in the judicial system though, wouldn't Digham have revealed Sullivan's murder in order to be redeemed back into the force?