And while she admits she was nervous in her first few scenes, that was nothing compared to her fears once the cameras were turned off.
Looking slightly sheepish, Charlotte explains that she daren’t go to the loo on her own while filming at Nonnatus House because she’s convinced that the toilets are haunted.
And not by phantom pregnancies.
“I can’t go in there alone,” she says. “When you go in you get this strange feeling that you’re not alone. It’s so spooky and surreal.”
Charlotte, 25, who recently moved into a flat in East London with a bunch of friends, can’t help being jumpy.
“I find Doctor Who really frightening,” she admits. “I’m a total scaredy cat.”
She insists she’s not the only one who thinks the show’s film set in Chertsey, Surrey, is haunted.
“The props guys told me they heard voices in the attic room when there was no one else up there. One came down and he was properly white,” she says.
She knows her fears are irrational but can’t banish them.
“Poltergeists really get me. I had a dream last night about a ghost. It’s not a question of belief – whether it’s true or not it definitely affects my life.
“For example, I’ve tried to watch Breaking Bad three times but because I watch it late at night it really traumatises me.
“I already have really weird dreams so I don’t want to make it any worse. After the episode where the man melts in the bath I realised I couldn’t watch it before bed any more.”
The nearest Charlotte has come to her own out-of-body experience is when she briefly “forgot” she was performing on stage. It was also one her most embarrassing moments.
She used to be one quarter of All Angels, a successful group of girl classical singers who played the 2011 Champions League Final at Wembley and in front of the Queen at the Albert Hall for the Festival of Remembrance.
At one such major gig, as she stood on stage at London’s vast O2 Arena in a spangly dress, she realised she had a wedgie down below. Quick as a flash, she hooked out the offending underwear, only realising afterwards that she had just done it in front of everyone.
“I got the wedgy sorted and then it dawned, Oh My God, there’s thousands of people here. I thought they must all be looking at the other three,” she laughs.
“I was so disassociated from myself I thought they were seeing the dress and the make-up but not actually me.”
After All Angels lost their record deal Charlotte forged a successful career as a TV actress, playing quirky Oregon in Channel 4’s Fresh Meat and hateful Hannah in BBC2’s Siblings.
Now she’s the keen-but-green Nurse Gilbert in Call the Midwife. She found she slotted in well to the “lovely” cast despite a few apprehensions when she joined following the departure of Jessica, who was the show’s main star.
“I was nervous about arriving as she left because I’d always watched the show with her in it. It was strange for me to see it without her, and that’s how I thought the fans would be feeling,” she says.
Nevertheless, this fourth series has enjoyed solid ratings of 8million despite the absence of Jessica and few sightings of Miranda Hart as Chummy.
Charlotte still can’t quite believe people want to see her on TV but also finds critical comments hard to take.
“When you’re on telly people say you’ve chosen to put yourself in my living room so why can’t I own you? Fair enough.
“But the flip side is that you’re really just trying to do the acting because it’s the one thing you think you’re good at.
“It’s not like, ‘Please criticise me.’
“On the surface you can’t take anything too much to heart but you should care, to a certain extent, if everyone on Twitter is going, ‘This is the worst thing she’s ever done.’ ”
Now she’s slowly learning to separate real life from her work. “In my personal life if anyone I know says anything bad about me, or if I feel I’ve upset someone, that kills me.
"But if I get a criticism of a performance from someone I don’t know, I just put it down to the character.
“Like someone said, ‘Why does Oregon walk like a man?’ and I thought, ‘Exactly – why does Oregon walk like a man? It’s not me.’ ”
Charlotte got into acting while studying for an English and drama degree at Bristol University. All actors approach their work differently, she says.
“Some people can just flip into it – go out all night and then just do it the next day. Others pore over their scripts for days and days and then can’t remember.”
She’s no fan of method acting, scoffing at the thought of Daniel Day Lewis being so immersed in his role as Abraham Lincoln he would only respond if called Sir or Mr Lincoln in real life.
“Imagine if you couldn’t switch it on and off,” she chortles. “I could see the appeal of changing my speech patterns for a year or something, but being Abraham Lincoln at the dinner table? That’s taking it too far.
“How on earth would you get him to take the bins out?”
That’s typically Charlotte, typically down-to-earth. She grew up in South London, the daughter of musical parents who met when dad Robin invited mum Elise to be a singer in his band.
The friends she made on her first day at school are still her best pals and her boyfriend is nothing to do with showbiz.
“He’s an economist,” she explains. “I don’t understand half of what he’s talking about but it’s nice that we occupy different worlds. I want to be the expert on what I do.
He predicted the recent collapse in Greece. Not great for them but nice that he got it right.”
This Sunday’s episode of Call The Midwife is the last of the series and Charlotte has just started filming a second series of Siblings.