'I personally call it a head-to-head female thriller,' is Morven Christie's elevator pitch for BBC's new psychological drama The Replacement. Christie plays Ellen an ambitious architect whose career is just taking off after landing her company a huge project building a new library. 'If you are an architect, you have to train for seven years so it becomes your whole world, it's the centre of her universe. She finds herself a bit lost on discovering she's pregnant; she has to accept changes to how she defines herself.'

The woman she's going 'head-to-head' with is Paula, played by Vicky McClure, who joins the firm as Ellen's maternity cover. At first she seems too good to be true, but soon starts to overstep the mark, ingratiating herself with her new boss and client, insinuating her way into Ellen's life. Does she really have a sinister agenda or is Ellen suffering from antenatal depression? 'There's a lot in the story about how much of what Ellen experiences is actually happening, and how much is her projecting her fears,' adds Christie.

With any thriller, the villain is crucial to the success of the entire show. With tough demanding roles in This is England, Line of Duty, Broadchurch and The Secret Agent McClure has proven her versatility. 'We did a lot of quite intensive prep before we started shooting,' explains Christie. 'We both have the same approach, we get all the hard work done in advance so when we get on set we can just put the pedal to the metal and go. So there wasn't a lot of faffing around, we'd go at the scenes full throttle. And then once they called "cut", we went off and screamed with laughter.'

Written and directed by Joe Ahearne – who has previously worked on Doctor Who, This Life and wrote Danny Boyle's 2013 feature Trance – The Replacement taps into universal fears about our place in the world, our sense of individuality, the idea that we can be exchanged or upgraded. 'It's the story about a specific period in a woman's life but really it's about identity,' says Christie. 'Joe's a big Hitchcock fan so he's very into the idea of being with one character and watching the world through their eyes. The Replacement is a thriller but my responsibility is to play a human being, the genre is what the director takes care of.'

When we talk, Christie is back home in Glasgow taking a break to recharge her batteries, hang out with her dog and attend a few gigs at this year's Celtic Connections, before heading down to the Lake District to film series two of The A-Word.

'When Joe wrote The Replacement, it was quite an anonymous city, but once it was greenlit, the idea of shooting in Scotland came up,' says Christie. 'He knew Glasgow so rather than Glasgow standing in for any city, he made it very specific, so they committed to a Scottish cast. I'm a native Weegie so it was great for me.' That Scottish cast also includes Dougray Scott, Richard Rankin and Neve McIntosh.

Mirroring her character, Christie's career has gone into overdrive over the last few years with lead roles in Grantchester, The Hunted and Twenty Twelve. Not forgetting a guest spot on Doctor Who. 'I'd never watched it, which is probably something I shouldn't admit, sci-fi is not my vibe at all. Actually it was a complete scream, just six weeks of silliness, we had such a laugh. I tend to be drawn to heavier dramatic stuff but sometimes you just want to run around corridors with aliens chasing you.'

The Replacement is on BBC One, Tue 28 Feb, 9pm.