The character in question is Bert, a KKK member who (spoiler alert!) is turned into a vampire. The role required the actor to say a racial slur, but mgk says he refused to do it.

Baker is likely referring to the character Bert, a KKK member who is turned by Remmick, the vampire portrayed by Jack O’Connell. In the final version of Sinners, the role is played by Peter Dreimanis.