Cult
The actress stars as Miss Foster in the first episode of the new series, alongside David Tennant and new companion Catherine Tate.
Her character, described as an "enigmatic and powerful businesswoman", leads an invasion of Earth by a secret army of aliens called the Adipose.
She told the Daily Star: "Miss Foster is, essentially, a nanny, a guardian to some rather unusual children. I see her as a slightly warped Mary Poppins. She's a strong woman.
"The first time I read the script I thought: 'Oh well, she's a baddy.' But the more I read it, I thought: 'No, she's doing what she's doing for legitimate reasons.'"
The new series begins on Saturday at 6:20pm on BBC One.
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