Millie Bobby Brown‘s characters are experts in sleuthing, but there’s one mystery the actress can’t touch in real life.

 Millie, 18, recently detailed the boundaries that exist within her “really good” friendship to Henry Cavill, her fellow Enola Holmes 2 actor. Talking to Deadline in an interview published Nov. 2, Millie said their relationship is unlike the “schoolmate”-like connection she shares with Stranger Things co-stars Noah Schnapp and Charlie Heaton.

“With Henry, it feels like a real adult relationship. Like a really healthy one,” Millie said, “One that we have terms and conditions. I know Henry. He has terms and conditions with me.”

And just what do these terms and conditions entail? For starters, the she noted, “I’m not allowed to ask about his personal life.”

“It’s like, ‘Millie, shut up. No.’ And I’m like ‘Understood.’ Whereas with the Stranger Things kids, it’s different,” she continued. “There’s no boundaries because it’s like we’re all siblings. But with Henry, he’s very strict with me, which I appreciate.”

Describing Henry as having a “very rational approach” to things like his character Sherlock, Enola Holmes 2 director Harry Bradbeer told the outlet that the Man of Steel actor and Millie had an “interesting, enjoyable dynamic” on set.

“They’re always poking at each other,” Harry said. “And Millie tickles him up a little bit and surprises him. And he goes with it.”

For her part, the Millie said she drew on her family interactions when it came to her relationship with the 39-year-old, who doesn’t have sisters of his own.

“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,” Millie said. “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.”