Warning: Thor: Ragnarok spoilers abound!

It was early 2016, and Eric Pearson was practically living at the Frank G. Wells Building on the Disney Studios lot in Burbank, camped out in his office until 2 or 3 o’clock in the morning. The screenwriter, who’d risen through the ranks of the Marvel Writer’s Program, was nearing completion on an early draft of a script for Thor: Ragnarok, the threequel currently thundering all over the worldwide box office. (The film just passed the $500 million mark.)

Pearson was nearing the climactic showdown between Thor (Chris Hemsworth) and the indomitable Asgardian goddess of death, Hela (Cate Blanchett), and the stakes didn’t seem high enough. That’s when the idea struck: What if Hela was actually Thor’s blood relative? Specifically, his sister.

In the comics, Hela is “allegedly” the daughter of Loki, Thor’s shape-shifting, trick-loving brother played by Tom Hiddleston in the movies, and the sorceress Angrboda. But that had never been the plan for the screen treatment. “We had decided that she’s like this ghost of Asgard’s past that’s come back,” Pearson explained to Yahoo Entertainment. “She represents the kind of violent way that they won their kingdom that Odin’s been trying to cover up.”

On one of those late nights, Pearson encountered Brad Winderbaum, VP of production and development at Marvel Studios who himself had climbed company ranks, first as a unit producer on Iron Man 3, then as a co-producer on Ant-Man, and now as an executive producer on Ragnarok. “I was like, ‘We’re getting to this moment, and it just doesn’t have the impact,‘” Pearson recalled telling him. “I’m like ‘With all this stuff, she should be Thor’s sister. And that should be the thing that [represents] what it is to rule Asgard, his family, what he’s been told, what he hasn’t been told.'”

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Winderbaum offered Pearson, who had previously contributed rewrites on Ant-Man and worked on various Marvel One Shot shorts but was getting his first go at a full feature, some sage advice. “Brad told me, ‘Don’t tell anyone. Just write it into the script. If we pitch it, it’s so much more likely to get shot down. Just write it into the movie.'”

Pearson initially gave the reveal to the scarred and indifferent Asgardian warrior Valkyrie (Tessa Thompson), who told Thor in Hulk’s suite, ‘She’s your sister.’ The script, mythology-busting twist included, then made its way to the desk of Marvel Studios chief Kevin Feige.

Pearson was terrified. “When they did that first read, I was like, ‘Oh man, if they don’t like it, we’re screwed.'”

Obviously, they liked it. “Brad showed me Kevin’s note and [the reveal] was circled with “WHOA!” [spelled out next to it],” Pearson recounted proudly. Eventually the revelation would be delivered via Odin (Anthony Hopkins), the cinematic father of Thor and Hela, because, as Pearson explained, they wanted to make the most out of the Asgardian king’s limited screen time and make Valkyrie’s role in the film less Thor-reliant (more on that below).

In the end, it was well worth the gamble. “I was just so happy that it played out because we were taking a risk,” said Pearson.

Some other notable nuggets from the writing (and rewriting) process Pearson provided:

There was almost a romance subplot with Thor and Valkyrie.

When Pearson took over scripting duties from original writers Craig Kyle and Christopher Yost, Thor and Valkyrie had cozied up. “But we didn’t want to start from that place,” Pearson explained. “It was like, let’s give Valkyrie her own story that connects with Thor… and if it makes sense for them to get together, then great. You’ve got two really good-looking people who can fight and who’d probably be [good together] if the story went there, but it just didn’t. It became more about the mutual respect, and also dealing with her PTSD. She’s someone who’s drowning her sorrows in the bottle, and I just thought that was such a cool thing that you don’t often see in these movies: somebody dealing with extreme guilt and shame in a colorful, Taika Waititi[-directed] hilarious background.”

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