On Tuesday, Salma Hayek shared a candid photo with Harington, which seems to indicate the pair has begun work on the film. She wrote alongside the fun image: "I still can't believe that I'm working with #jonsnow ! Kit you're the best!!!"
In the image, 32-year-old Harington, wearing a black T-shirt under a white jacket, cracks a big smile while wrapping his arm around his new co-star. Hayek, 53, grins by his side in a dark-blue blouse and jacket.
The film, which started shooting this summer, has largely been kept under wraps but we know that it will center on the titular race of immortal aliens, created by the Celestials, who protect humanity from their villainous counterparts, the Deviants. Hayek will play Ajak, the leader of the Eternals while Harington is taking on Dane Whitman, aka the Black Night.
ET spoke with Hayek after the film's presentation at Disney's D23 Expo in Anaheim, California, in August, where she shared her enthusiasm for the massive project -- and her ability to finally talk about it.
"I've known for a long time. I've known since March," said Hayek alongside co-stars Lia McHugh and Ma Dong-seok, of her casting. "We have to keep it a secret and it kills you. It's like the most amazing thing that can happen to you, but you cannot tell anyone."
Hayek also shared that, in the comics, her character is a man. However, the film's creators decided to give Ajak a more motherly role.
"But Chloé [Zhao], the director, has approached the leadership a little bit more [with a] deeper connection," she explained. "So, it's like a family and I'm like their mother, and they're all beautiful in different ways."
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Salma Hayek Says Her Husband Is 'the Most Excited' About Her Role in 'The Eternals' (Exclusive)
When asked what Hayek's 11-year-old daughter, Valentina, thinks of her being a superhero, Hayek joked, "Maybe she will listen to me now?!"
The film also stars Richard Madden, Kumail Nanjiani, Lauren Ridloff, Brian Tyree Henry, Don Lee, Barry Keoghan, Gemma Chan and Angelina Jolie, who admitted to ET at D23 that the production is so secretive, she had no idea Harington had joined the cast until it was announced on stage!
"I'm not so sure what we can say," Jolie said with a smile. "I still think we're seeing each other behind the scenes, like, 'Did you hear...?' I didn't know Kit was in the film! It's a whole thing. I'm thrilled, but I had no idea."
The Eternals hits theaters Nov. 6, 2020.
See more Marvel updates from D23 in the clip below.