Michael Fassbender cannot lie — the polygraph scene in Black Bag was a bear for him to shoot.

Not because he had to pass the test (in fact, his character, George, is the one administering it). But on the page, screenwriter David Koepp wrote the scene as we see it in the film, cross-cutting between George interrogating all of his colleagues, including Clarissa Dubose (Marisa Abela), Dr. Zoe Vaughan (Naomie Harris), Freddie Smalls (Tom Burke), and James Stowell (Regé-Jean Page).

However, when it came time to shoot the scene, director Steven Soderbergh wanted to approach it in a more linear fashion, capturing all of one character’s coverage at a time. “That was definitely the most challenging scene for me because in the script, it jumps between the characters as it’s written dialogue-wise,” Fassbender explains. “So that’s the way I learned it. But then on the day Steven was like, ‘You’re just going to do all of Clarissa’s dialogue in one, and you’re going to do all of Stokes and all of Freddie.'”

“I hadn’t learned it that way, and I’m asking quite similar questions to each of the characters,” the actor continues. “But there’s subtle differences there. I was panicking. I had the pages all over the place in front of me.”

Soderbergh confesses that he hadn’t meant to trip Fassbender up; he was merely trying to shoot the scene most efficiently. “It makes perfect sense if you’re on a normal movie to memorize the script as written,” he jokes. “It just turned out in order to do this most efficiently, everybody had to be shot from these four different angles.”

“I would set up two of them at a time and just rotate them in and out,” Soderbergh continues. “And as soon as I explained that to Michael, he went, ‘Oh s—.’ He had to scramble his brain and get that sorted out.”

The scene was also quite challenging for Abela, whose Clarissa is a whiz at beating lie detector tests. “I watched a lot of videos of people taking polygraph tests,” she says of her preparation. “It’s about managing your own breath and your anxieties. Even things like getting in a cold plunge and making my breath and my heart rate try and come down to a normal place, that was the closest I could get to managing what it would feel like to be in a high-stress situation and come off as cool, calm, and collected as possible. I read lots of accounts of what they’re actually looking for, like a rise in your heart rate and your pulse. So it was just about imagining what it would feel like if I was trying to keep all of those things at bay.”

While experts have questioned the validity and accuracy of the polygraph test, particularly because it is possible to trick the machine, Soderbergh says the device is used in espionage regularly and is far more sophisticated than audiences might assume.

“The intelligence agencies still use it a lot,” he says. “The metrics they’re using are pretty precise. We had a guy who does this for those kinds of agencies, and he felt very, very confident in his ability to judge whether somebody is lying. He’s like, ‘Look, can it be beaten? Yes, but I can tell you it’s really f—ing hard.’ You’ve got to be a sort of sociopath to really fool it because they’re collecting all this data — there’s a hand thing, heart rate, blood pressure, pupil dilation, it’s full on.”

The scene was full-on for Fassbender, too, but Soderbergh predominantly focused on all of the other actors first. “It didn’t matter because the camera’s hardly on me anyway,” Fassbender says, with a laugh. “I wish he’d told me that, but yeah, I was panicking a little bit that day.” 

Want more movie news? Sign up for Entertainment Weekly‘s free newsletter to get the latest trailers, celebrity interviews, film reviews, and more.

However, in Soderbergh’s eyes, it all went exactly as it should have. “I shot all them out first, and then I came and shot Michael, and then, he got to do the version of it that he had memorized,” he explains. “So it all worked out, but there was a little minute of, ‘Wait, what are you doing?'”