Charlotte Ritchie on adorable dogs, Siblings and the future of BBC3

"She is sociopathic," Ritchie tells us. "She is fully, fully driven by her own desire. Something really awful happens this series and her reaction is staggeringly cold-hearted."

Hannah's the polar opposite of her brother Dan, played by Tom Stourton, who you might recognise from Doctor Who and Horrible Histories. "Dan has this sweetness in him. It’s just this wide-eyed loneliness. He’s so desperate for friends and so open. It’s astoundingly funny," she says.

"He has a very low IQ and a big heart!" laughs Stourton, when we catch up with him. He's more relatable - "If I wasn't acting I would be Dan. I'd be Dan but without the sweet flat with my sister, because my sister's married and has a nice house with her husband..." - but just as destructive.

"I ruin someone's life. Someone gets set on fire. There's a pregnancy, a music video and a kidnapping. It's definitely more ambitious," he tells us, of series two's dramas.

"A lot of the comedy lies in seeing this bulldozer go through situations and just cause absolute chaos," agrees Ritchie.

For her, Siblings is a culture shock from Call the Midwife. "It felt relentless, in a really good way. It felt like we went from scene to scene to scene. There was so much dumb stuff we got to do. It’s quite liberating."

It's "freer" than Midwife, she tells us, but also more pressurised. "I normally have one or two lines in a couple of pages of dialogue. Siblings is faster and I have a lot more lines. We might do seven or eight scenes and I'm in all of them."

And there are also no adorable dogs on set, while Midwife is practically a part-time kennel, filled with the cast's pets.

"Nothing can go wrong when you've got dogs around. They just make you feel calm," says Ritchie, who admits she's tempted to get her own: "If I wasn't afraid of the stigma of having a handbag dog I would get one. Definitely."

"Dog's have their priorities right. They know what they want. Life is simple for them," she laughs.

Life for Ritchie and Stourton's sitcom is a little more complicated at the moment, in light of changes at the BBC. Both of the show's stars are uncertain about what BBC3's rebrand and move online means for Siblings and a potential third run. But they are sure the youth channel should be protected.

"It’s important to me. I watch loads of stuff on BBC3. Not just Siblings!" Stourton says, adding: "I hope it doesn't, but if they are cutting the channel for budget reasons it’s hard to see how that won’t effect the budgets of the shows and the quality of what they put out there."

"BBC3's been a home for fledgling comedies," adds Ritchie. "They have a space and a legitimacy. You can find all sorts online but something is taken more seriously if it's on telly."

When it comes to the future for Siblings, "it would be a shame if it didn't air on TV," says Ritchie.

"So many people catch up on iPlayer - it's a really strong place for it to exist - but the worry would be that people wouldn't know it was there."

Siblings series two starts tonight at 10:30pm on BBC3