In an exclusive behind-the-scenes look at the film -- out on Blu-ray and DVD September 12 -- Boutella explains how Cruise popped up out of nowhere for their first screen test.
"We were in London and [Director Alex Kurtzman] said can we just improvise because they needed to see facial features and all of a sudden, camera right, Tom just popped on the side and I've never acted with him," she said.
The footage from the set shows the two getting intimate with each other in character as Ahmanet and Nick Morton.
"Sofia, she brings a real exotic charm, an exotic energy and commanding performance," Cruise said of his co-star.
Boutella also admitted she was skeptical of taking on the title role at first, fearing she'd be trapped under extreme makeup two films in a row.
"When I read the script first, I was finishing 'Star Trek' and there was also prosthetics for the character and it terrified me being back with loads of prosthetics being put on me," she said. "Alex talked about the character and he wanted things to come from her more delicate instead of monster like, she's a queen. He showed me storyboards with my face on it and I was like, why would I say no?"
See some of those early storyboards in the video above. "The Mummy" hits Blu-ray/DVD on September 12.