Spoilers have always been an issue for the Marvel Cinematic Universe, from Tom Holland livestreaming a poster clearly labeled “CONFIDENTIAL: DO NOT SHARE” to Andrew Garfield accidentally spoiling his appearance in Spider-Man: No Way Home by tipping off a delivery driver. You can keep set details under wraps, but actors are human, and there’s no telling what they’ll do.

With Avengers: Doomsday, Marvel is assembling a massive cast of Avengers, X-Men, and everyone in between to fight Robert Downey, Jr. as Doctor Doom. You’d think having so many actors involved would make secrets difficult to keep, but a quirk of Marvel’s production is keeping them in the dark. No one knows anything because, even though cameras are rolling, the script is still being written.

Rebecca Romijn doesn’t know how much of Mystique is in Avengers: Doomsday.

Rebecca Romijn, who played Mystique in the original Fox X-Men trilogy, told The Hollywood Reporter that she doesn’t know what the extent of her appearance in Avengers: Doomsday will be. “The script hasn’t — they haven’t finished writing it,” she said. “It’s been very, very fun, and we don’t know yet. They keep everything very close to the vest themselves in an effort to keep everything under wraps.”

The commitment to secrecy is admirable, but finishing the script well into production is a bad habit for the MCU. While drafts are written before cameras roll, changing or adding elements — called “plus-ing” — is common. At a press event earlier this month, Kevin Feige revealed that it’s a constant process. “We’ve never started a movie without a full script, and I have never been satisfied with a script that we’ve had,” he told Variety.

Now, Avengers: Doomsday is getting the same treatment, with Loki writer Michael Waldron assisting Stephen McFeely with the script while production continues in London. “There’s plus-ing happening every day on the Avengers: Doomsday set right now,” he said at the same event. “It is amazing to watch because what those filmmakers, those actors, both the ones that are playing these characters for the first or second time and the ones playing them for the 10th or 12th time, are the best in the world at it, and know these characters so well.”

Kevin Feige stands by the practice of tweaking scripts during production.

It may be a common way to make high-profile superhero movies, but that doesn’t mean it’s the only way. James Gunn, a veteran Marvel director and current co-president of DC Studios, told Rolling Stone that “the movie industry is dying” because movies are getting made with unfinished scripts. Gunn even shut down development of an unnamed DC movie because the script wasn’t satisfactory, and plus-ing couldn’t be counted on to solve it.

Gunn is a meticulous director who avoids reshoots whenever possible; Superman required just a day and a half of them. Should that become the practice for Marvel? That might not be possible with a movie as big as Doomsday, but Gunn has proven that you can make a superhero blockbuster with a finished screenplay from the get-go. Hopefully, Marvel can learn the same lesson. Rewrites aren’t inherently bad, but at a certain point, you want your actors to know just what it is they’re doing in front of the cameras… even if they accidentally spoil something.