The 18-year-old actress said that while it felt like her and her young Stranger Things co-stars were "all siblings" on the hit Netflix sci-fi shows, with Man Of Steel star Cavill, 39, it was totally different.
“I grew up with [Stranger Things stars] Noah [Schnapp] and Charlie [Heaton]," she told Deadline.
"I met them when I was 10, so for me it does feel like your schoolmates.
"With Henry, it feels like a real adult relationship. Like a really good friendship, a really healthy one.
“I know Henry. He has terms and conditions with me,” she continued.
“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.
“There’s no boundaries because it’s like we’re all siblings.
"But with Henry, he’s very strict with me, which I appreciate.”
The actress also believes that because the actor didn’t grow up with sister’s it was tricky for him to know how to deal with her as a pesky teen.
"He doesn’t have any sisters. So, for him, it must be completely different," she explained.
"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."
Brown and Cavill recently attended the red carpet premiere for Enola Holmes 2, which was released on Netflix today, Friday, November 4.
It focuses on the next phase of Enola Holmes’ budding detective career, with the first installment being released back in 2020.