In a scene at the end of the sequel starring Eddie Redmayne, Jude Law, Zoe Kravitz, Ezra Miller and Johnny Depp, the true identity of Miller's character Credence Barebone is revealed to be Aurelius Dumbledore (presumed to be Albus's brother).
"I was shocked, stunned, confused, petrificus totalus with wonder and fascination," Miller told Entertainment Weekly. "And I still have a lot of questions for which I have no answers."
The 26-year-old actor said he first learned about his character's identity when the Fantastic Beasts team was promoting the release of the first film in 2016, with Rowling providing him with some early details.
Miller said that the knowledge he is a Dumbledore caused him to adjust his performance: "On a certain, very, very subtle level."
"This character is changing, going through physical changes in relation to the Obscurial [with] which we know he has an unusual relationship," he continued. "So as he changes, there's an opportunity for me as an actor to look to five of my favourite performances of all time – the people I've seen play Albus and Aberforth.
"Shout out to all those incredible actors, and I'm thrilled the play the game that we're now blood relations. It's ery fun. And there's a funny connection to the name Aurelius given one of the actors who played Dumbledore."
He added that Law, who plays Albus Dumbledore in The Crimes of Grindewald, is the only one who knows whether his character is aware that Credence is his brother.
Fantastic Beasts: The Crimes of Grindewald has received mixed reviews from critics, and recently marked a new low for Harry Potter films in the US, grossing $62.2m over the weekend.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies