Marvel’s Black Panther may have been named after Chadwick Boseman’s titular superhero, but the character who left the biggest impression was the proud, ambitious, and chillingly calculated Killmonger. In a new interview, actor Michael B. Jordan revealed a secret weapon that allowed him to slip into Killmonger’s head, and it’s eerily similar to how the late Heath Ledger became The Joker in the 2008 epic The Dark Knight.

On Tuesday, Jordan paid a visit to The Late Show with Stephen Colbert to promote his HBO movie Farenheit 451, a live-action adaptation of Ray Bradbury’s dystopian classic. When the subject of Black Panther came up, Jordan said he kept a “dark” diary of Killmonger’s life, chronicling his childhood all the way up to his first appearance on the screen.

“I have journals, or diaries, or whatever, for every character that I have, from the earliest memory up until the first page of the script,” Jordan told Colbert. “And for me, it just kind of gives you context.”

Though Jordan said he has journals for his other roles like Adonis Creed (Creed) and the Human Torch (Fantastic Four), Jordan revealed that Killmonger’s entries were “dark” and “very sad,” with content “not right for television.”

“Not having his mom growing up, in and out of the system, foster care, foster homes and whatnot. It’s a lot of dark stuff that’s probably not right for television. But it was deep, and it allowed me to kind of go to that place right before a scene. It allowed me to lock in.”

Keeping journals is a popular method of study most (great) actors do, but there is an immediate similarity to the last king of movie villains: The Joker, played by the late Heath Ledger prior to his death for Christopher Nolan’s The Dark Knight. Ledger famously approached the role to an unbelievable degree, which involved locking himself in a hotel room for a month and spending his forming the Joker’s voice and psyche. He also kept a diary, telling Empire in 2007:

“It’s a combination of reading all the comic books I could that were relevant to the script and then just closing my eyes and meditating on it. I sat around in a hotel room in London for about a month, locked myself away, formed a little diary and experimented with voices — it was important to try to find a somewhat iconic voice and laugh. I ended up landing more in the realm of a psychopath — someone with very little to no conscience towards his acts. He’s just an absolute sociopath, a cold-blooded, mass-murdering clown.”

In the 2013 documentary Too Young to Die, the filmmakers perused through Ledger’s diaries, and naturally, the content was unsettling.

In the pantheon of great movie roles, Jordan’s Killmonger and Ledger’s Joker will go down as some of the finest ever put to cinema. While the two characters from Marvel and DC couldn’t be further apart, it’s no surprise their acclaimed actors shared something in common.

Black Panther is now available on Digital HD. It will be available on Blu-ray on May 15.

Photos via Marvel Entertainment