It's a hint of complexities that become more apparent in Call the Midwife's second season, which screened in the UK earlier this year and started last week on TV One.
The new episodes feature Sister Bernadette's first onscreen delivery: "It's a challenging birth, and so it was one of my harder-slash-better days on set, you know: something to get my teeth into." The episodes see her character go on "quite a journey", with a destination that kept Main herself guessing until the end.
"We usually get the scripts for only two episodes at a time, and although there were indications of where it might head, I was never entirely sure how things would pan out. It's a very, very slow burn.
"My concern was - and this gets into spoiler territory..." begins Main, before our phone connection is abruptly cut off.
After the call's re-established, we jokingly agree the interruption is a sign we shouldn't discuss any more plot points and turn instead to how Call the Midwife has unexpectedly made Main into a chart-topping recording artist.
"One of the many wonderful things about the show," she says, "is that we're now at a stage where [writer] Heidi Thomas says it's like having a fantastic repertory company of actors she knows so well that she's writing for us with our voices in mind."
In Main's case, that's more true than for most. A trained soprano who started performing as a child in musical theatre, the quality of her singing was recognised during the first season, leading Thomas to script more solos for Sister Bernadette in season two. These pieces, along with all the other key songs and incidental music from the series to date, were released on Call the Midwife: the Album, which rose to number two on the UK compilation album charts when it was released in February.
"That was a nice surprise and an extra bonus," laughs Main. "The best thing, though, is everybody on the team cares so much about these stories and they're made with so much love. It's a really happy place to work. And because it has done well, now we're onto making season three. It's great."
Season two of Call the Midwife is screening Sundays, 8.30pm on TV One.