Kelly appeared to be referring to the character of Bert, played by Peter Dreimanis. Married to Lola Kirke's Joan, the couple makes the mistake of welcoming Remmick (Jack O'Connell) into their home ...

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.