The X Factor judge and Radio 1 DJ appear alongside a cast including ex-Hollyoaks actress Emma Rigby, Misfits's Robert Sheehan and Outcasts actor Ashley Walters.
Directed by Arjun Rose, the film follows a group of misfits who form a suicide pact over the internet. However, each finds themselves in danger as they are singled out and murdered by a masked killer.
Teasing her role as Amber, Contostavlos told the Daily Mail: "She's very up and down, very on the edge - anything can make her tip. She's vulnerable and cute and warm, but at the same time she has a bit of a psychotic edge to her."
The 22-year-old, who previously appeared in Channel 4 interactive series Dubplate Drama, will also play singer Shanikwa in forthcoming comedy Big Fat Gypsy Gangster.
"I've always wanted to do a real kicking butt role. A bit of a Lara Croft, Tomb Raider... I love the Resident Evil characters," the N-Dubz star said. "Being able to learn a fighting skill and having to do training... just beating people up and I've got guns in holsters on my thighs which I pull out."
Demons Never Die opens in cinemas on October 28th.
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