The new show, which is based on John Wyndham's 1957 sci-fi novel, follows a small and quiet commuter town which is suddenly plunged into panic one day after a mysterious blackout that leaves every woman of child-bearing age in the town inexplicably pregnant.
WATCH: What do you think of the new show?
While the children appear to be normal, the residents soon learn that there's something much darker afoot.
Chatting to HELLO! and other journalists at a press screening of the programme on Tuesday, the Bodyguard star revealed that the actors playing the cuckoos would wear fake teeth to unify the characters in a subtle but sinister way.
"The make-up department did a brilliant job with the children. They all had much longer make-up calls than any of the adults," said the 46-year-old actress. "They were dealing with wigs and lots of little boosters, piles of boosters everywhere had to go on the make-up chairs, and the teeth.
Keeley revealed that the children wear fake teeth in the drama
"Amy, our brilliant makeup designer, had the idea that kids of that age are sort of scrappy and the thing that might unify them all is to have these perfect teeth."
She continued: "So it's not something that leaps out at you, it's not something obvious but when you're watching you just get the sense that something is not quite right because you're used to seeing children of that age with mangled teeth one way or another."
The series is available to stream on Sky Max and NOW
"It was a huge feat that alone, children's teeth are changing all the time, children are changing all the time at that age, so they did a really wonderful job of creating that image," she added.
All seven episodes of The Midwich Cuckoos are now streaming on Sky Max and NOW.
Like this story? Sign up to our What to Watch newsletter to get other stories like this delivered straight to your inbox.