Millie Bobby Brown and Louis Partridge on belly laughs, sibling vibes and Enola Holmes

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."

The third film takes Enola, the teenage sister of Sherlock Holmes, to Malta, where her future with Tewkesbury is interrupted by a dangerous new case involving Sherlock's disappearance.

This chapter pushes the franchise into darker, more grown-up territory which was part of the appeal for new director Philip Barantini, best known for Adolescence and Boiling Point.

"My nine-year-old daughter can't watch anything I've ever done," he says. "She's a big fan of Enola, and I wanted to make something we could watch together and challenge myself to do something different."

His job was made easier by the fact that Brown - who has been a producer on the franchise from the start - was already thinking along similar lines about the direction of the film.

"When I pitched my idea, she was already thinking about incorporating some of the darkness so it made my job easy because we were so aligned."

The film also gives Himesh Patel a proper introduction as Dr Watson, after a brief cameo at the end of the second film.

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."

Brown is keen to draw a distinction between caring about detail and picking a film apart for the sake of it.

After first-look images from the film were released, some fans questioned whether Enola's nails looked too modern for the period setting.

"How bleak and boring of the internet, I love a good manicure and so does Enola."

She sees it as part of a wider habit of people picking things apart online.

"I wasn't disappointed but I was like, oh OK, that's what the articles are about. But then again, the internet does not surprise me these days.

"I've been through it on the internet."

Barantini was more amused by the reaction and says he doesn't think "small things like that matter".

"Maybe they do for some people but it made me laugh. We just live in a world where everyone is hyper-aware and they find something to zone in on, and it becomes a huge thing."

"Hopefully in years to come that will change and we can uplift boys and girls," Brown says.