In the new four-part BBC One thriller, she portrays pivotal character Emma Matthews, who moves into a minimalist and sinister house designed by David Oyelowo's Edward.
And although she considered this particular opportunity to be unmissable, Plummer recently admitted to RadioTimes that it took her out of her comfort zone.
"We shot it in Bristol, and I remember that the first week I was there, it was like, 'Oh my God, this is probably how I felt when I was on my first day of school,'" she recalled.
"The whole casting was done over Zoom, so I hadn't actually met anybody face-to-face until we were in the thick of it. So it's very, very daunting for sure.
"In EastEnders, although you might be on different teams, you know everybody, so it's familiar. And as much as that familiarity with everybody is comforting and assuring, being on something like this with brand new people, it's just another opportunity to learn new things."
Elaborating on this mentality, Plummer went on to explain: "I find it really interesting to meet... not necessarily actors, but you know – we became such a family from the camera department to the make-up department.
"It just felt like we were all small parts of a puzzle that connected made the most beautiful picture and yeah, it didn't take long before I felt really comfortable with everybody."
Elsewhere in the show's cast, Loki star Gugu Mbatha-Raw plays Jane Cavendish and fellow Albert Square face Ben Hardy is Simon.
The Girl Before airs on BBC One.
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