It is rare to meet two co-stars whose chemistry is obvious before the interview has even begun but before I ask Millie Bobby Brown and Louis Partridge a single question, they are already laughing together.

As they return for Enola Holmes 3, they say that off-screen friendship has helped them bring more ease and affection to Enola and Tewkesbury's developing relationship.

"We realised we don't do much prep for some of the scenes - we learn our lines and rehearse and it's not that we don't take it seriously but we're really able to be in it," Partridge says.

Brown laughs as she jumps in, turning to Partridge with the kind of mock-seriousness that suggests this is a familiar routine between them.

She says they bring out each other's sillier side and they spend most of their time on set "belly laughing about the most random things".

"People will say to me, 'I just met Louis Partridge and I had a really interesting conversation, he's a really smart guy. I'm like, who? Louis Partridge is a smart guy? When?" We definitely regress into our younger selves."

Partridge, 23, smiles at this and tells me there is something almost sibling-like about their dynamic, which may explain why they are so comfortable teasing each other.

"We were just remarking that we look like brother and sister," he says. "There's definitely a bit of that relationship between us."

Enola stays with her so much that Brown's husband, Jake Bongiovi, who also worked on the film, says she sometimes takes the character home with her.

Brown agrees that Enola definitely stays with her and jokes that the teen detective's high standards can also follow her home.

"Enola is high maintenance and sometimes I go home and I'm like, 'Why does the bed look like that?' Then I'm like, Millie, chill, you do not need to speak like that."

What she is most protective of, she says, is Enola's intelligence.

Partridge remembers filming once stopping "for five hours" because Brown felt Enola would have worked something out sooner.

"She was like, no, no, no, Enola would know that flag is from this country and she would have worked that out."

Brown cuts in, laughing. "Oh please, it was not five hours," she says. Partridge revises it to three, but Brown is still not having it.

"He's such a liar, it was not three hours," she says.

She insists she is usually "the biggest girl on getting things done quickly", but says this mattered.

"Like those fans who critique absolutely everything, I want to make sure I'm bringing to life a plot that makes sense and characters that people can believe in."