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The last time actress and singer Charlotte Ritchie had to do research for a role it involved finding her way around a bong.

But as new kid on the ward in the BBC's hit period drama Call the Midwife, the 25-year-old faces a rather different prospect.

"All my friends were at Glastonbury - but I was at home watching a woman giving birth on YouTube with the Glasto highlights on mute," Ritchie told the Mail on Sunday's Event magazine.

"When I was filming (Channel 4 student dramedy) Fresh Meat, the only piece of equipment I had to research was a bong and how to smoke it; now it's forceps and a girdle."

Ritchie joins the cast as a Liverpool clergyman's daughter, Barbara Gilbert, in the new series of Call the Midwife, which begins airing tonight.

She has described the Nonnatus House newcomer as a selfless soul who wears her heart on her sleeve: "In an era when emotions were supposed to be kept under wraps, her inability to do that is endearing. It's a shortcoming as much as a strength."

She said it took a turn in starched 1950s midwives' blues before she truly believed she was part of the show - in fact, Ritchie's own mother was so convinced her casting had been a case of mistaken identity she would not let her share the news for a week.

"The uniforms are fitted to you standing up," Ritchie revealed.

"They are so stiff and the collar so tight that you simply can't slouch. Underneath we wear a period bra, one of the pointed ones. We should technically be wearing stockings and suspenders but we're allowed to wear tights, and then on go those sweet little lace-up shoes. They make the biggest difference. Those clicky heels make you feel prim and proper. The shoes are everything ...

"It was only really once I was in uniform that I realised I'd actually got the part."

Though Jessica Raine's Jenny Lee will be missing from this series, stalwarts including Helen George (who plays midwife Trixie Franklin) and Laura Main (lapsed nun Shelagh Turner) remain.

George has revealed her character will take centre stage this season as she falls for parish priest Tom Hereward (played by Jack Ashton) - and there will be a wedding.

But she is keeping mum on the details.

"There's a wedding of some sort. I'm not saying who with but there is a marital ceremony," she told Event.

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