Who is pupil midwife Nancy Corrigan? Meet Call the Midwife newcomer Megan Cusack

As fans will recall, Sister Julienne (Jenny Agutter) has been tearing her hair out trying to find a way to secure Nonnatus House's financial future; but Nurse Shelagh Turner (Laura Main) has now come up with a brilliant plan, securing funding by creating placements at Nonnatus House for four trainee midwives.

Three of the pupil midwives live off-site – but Nancy, an orphan brought up by a Catholic Order of nuns in Ireland, is boarding at Nonnatus. And whether she's a permanent addition to the cast or not, it sounds like she's got quite the story arc coming up in season 10.

"There's more bubbling underneath the surface than you see in that first episode, which is lovely," Cusack says. "It's nice to play a character that has an arc, and to see where she goes... she goes through some tough times, there are some big revelations and I think emotionally she goes on quite a journey. She deals with some stuff, does Nancy."

Nancy is a talker, but that's not appreciated by all of her patients. Cusack explains: "She's a bit ditzy. She's erratic, not great with her studies, but she's got a big heart, very warm, she'll chat to everyone. Her chat doesn't necessarily always make people feel good, she makes people nervous I think sometimes with how she talks. She doesn't have a filter, which I quite like. She doesn't think before she speaks."

As for her backstory, Cusack tells us that Nancy lived with her grandmother after she lost her parents – but when her grandmother passed away, she ended up in the orphanage. Moving to London gives her a chance to try out the bold fashions she loves, and to come into her own. However, she's still finding her way. "She's big and she's bold, but underneath that she's still a very young girl who just probably just wants people to like her really," says Cusack. "She wants that warmth from people."

Cusack herself is a relative newcomer to the acting world, having graduated from LAMDA and then picked up roles in Doctors and The Cherry Orchard – but Call the Midwife will be her first big break.

However, she does have a few family members she can call up for acting tips; if the surname sounds familiar, that's because she's from the Cusack acting dynasty in Ireland, including aunts Niamh Cusack, Sinéad Cusack and Sorcha Cusack. Her grandparents were Cyril and Maureen Cusack, and her father is the theatre producer Pádraig Cusack. Oh, and her cousin is Max Irons.

"They've been vital at times with auditions," Megan Cusack says. "They've especially helped me when prepping for auditions and stuff like that. Though my mum always says, 'Be a plumber, an electrician, or a hairdresser, because they're always in work.' But obviously I didn't listen.

"They've all said from the beginning it's really tough. It's a tough industry and it's a tough business and it's a tough career. So you've got to really want it." And as for the advice they've given, it's "probably just to enjoy it. Try not to get in your head too much when you're going in, because the more you're having fun, the more other people will enjoy what you're doing. And always do your prep. Just always be ready!"

Cusack does have at least one relative who is not in the producing or acting business, however, and that is her grandmother – who actually used to be a midwife, and is now a devotee of Call the Midwife.

"Well so, my grandma is a big fan, because she was a midwife herself," Cusack says. "I've seen bits with her, which was cool. but then she always talks through it. 'Oh, I remember this.' It's like, yeah! They're talking, Grandma!"

Grandma Margaret is on of the only people who's known the secret about Megan's casting up until now – and she's "delighted", Cusack says. "She keeps ringing up my mum. She'll be like, 'Call the Midwife's on! Is this one going to be one that Megan's in?' And mum's like, 'No Margaret, because Megan's not done filming yet. I'll let you know, I'm going to record it for you.' And she's like, 'Well I dunno – she could be!'"

Cusack's casting as Nancy came about very suddenly.

"I got an audition through from my agent," she explains. "I had a self-tape. And it was so quick-paced: I got it on the Friday, filmed it on the Sunday, sent it off on the Monday, and then got the call on the Wednesday saying that I'd got it. So it was a bit mental. And then the next thing, I was in getting hair and fittings and being measured. And then the week after, it started. So it was definitely a whirlwind."

She add: "I actually remember the first moment that I was like, this is what I - I'm doing this, this is mental... I remember standing waiting with the two suitcases, waiting to go in, and I had this real feeling of like, 'Oh my God this is what - I'm doing this now! This is my job for the next few months, and I'm playing this character, and this is all a bit mental.' But really lovely at the same time. It was like a sudden overwhelming feeling, it was probably my third day, and it hadn't quite sunk in yet."

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Joining a show already in its 10th year was, she admits, "slightly intimidating" – "but as soon as I got there, everyone was so welcoming... it's definitely much much more than what I expected it to be. I went in quite nervous and I mean, as most people I think would be, and I just had such a great time. I've just really enjoyed it. It doesn't really feel like work, and everyone's been so gorgeous I couldn't really ask for anything more."

Call the Midwife continues on Sundays at 8pm on BBC One. While you're waiting, take a look at the rest of our Drama coverage, or check out our TV guide.