In contrast, her friendships with “Stranger Things” costars like Noah Schnapp and Charlie Heaton have fewer boundaries due to the cast growing up with one another. “It’s different because I grew up with Noah and Charlie. I met them when I was 10. So, for me, it does feel like your schoolmates,” she said.

Meanwhile, the 39-year-old Cavill is more of a big brother to the “Enola Holmes” star — one with a very strict rule about his private life. “I’m not allowed to ask about his personal life,” Brown revealed. “It’s like, ‘Millie, shut up. No.’ And I’m like ‘Understood.’ Whereas with the ‘Stranger Things’ kids, it’s different. There’s no boundaries because it’s like we’re all siblings. But with Henry, he’s very strict with me, which I appreciate.”

If anything, the level of professionalism they maintain has only made their working relationship stronger. “Enola Holmes 2” director Harry Bradbeer shared with Deadline that the actors’ differing personalities help make their onscreen brother-sister relationship as Enola and Sherlock Holmes feel all the more authentic.

“In a way, Henry, like Sherlock, has a very rational approach but is learning more and more to have fun and to be spontaneous,” Bradbeer said. “And of course, Millie and he had this very interesting, enjoyable dynamic where they’re always poking at each other. And Millie tickles him up a little bit and surprises him. And he goes with it.”

For his part, Cavill doesn’t have any sisters, and Brown has taken it upon herself to give him the full little sister experience while they’re filming. “It’s shocking because he doesn’t have any sisters,” she said. “So, for him, it must be completely different. I have a sister, a brother, and an older brother. So, I’m very much used to that kind of dynamic. I had to implement a lot of that into my relationship with Henry offscreen and onscreen and just pull him out of his comfort zone.”