While visiting The Late Show with Stephen Colbert to promote his upcoming HBO film Fahrenheit 451, Michael B. Jordan revealed that he kept a diary to help him drop into the mindset of Black Panther villain Killmonger.

“I write journals for every character that I have, from the earliest memory up until the first page of the script,” said Jordan during the interview which aired Tuesday, May 8. “For me, it just kind of gives you context.”

The diary for Killmonger, said Jordan, “was particularly dark. It was a very sad journal, not having his mom growing up, (going) in and out of the system (in) foster care, foster homes and whatnot.”

Parts of the diary were too dark to share, with Jordan saying it’s “probably not right” to talk about it, but he did say it helped him “lock in” to the character.

Fahrenheit 451 airs on HBO May 19.