Ruth Wilson has promised that her psychopathic character Alice Morgan will be "back with a vengeance" to kill more people in the fifth season of Luther.
The British actress - most recently seen playing her own grandmother in the BBC drama Mrs Wilson - first appeared in Luther's initial episode in 2010, but fans feared her character was dead at the end of the show's fourth run.
Wilson said: "We find that Alice isn't actually dead. Things have all gone a bit wrong, so she's come back to get Luther's help.
"It's an action-packed series. Alice is back with a vengeance. She's funnier than ever; she's more lethal than ever and she and Luther get up to some dastardly things.
"For Alice, unlike a lot of the characters that I do, I think it's about having joy in the scenes and finding the moments of levity and playing around with the scenes that exist on the page. I think it's about having a freedom to play a bit more.
"She doesn't feel deeply. Or she doesn't go there. She's able to avoid feeling deeply so she doesn't act with conscience. And it's quite nice to play.
"It means I don't have to worry about the subtext of what's going on underneath her, it's almost like water off a duck's back. So it's quite liberating to play her, a joy to play her."
The new season reunites Wilson with Idris Elba in the title role of John Luther, and she said: "We have such a rapport and we worked out it's been almost 10 years since we started the first season.
"We've got a shorthand and it just becomes more quirky as we go on. We are very at ease with each other and the scenes become so much more than what is on the page because we can play."
Elba said the audience will get to know Luther more in the fifth season, adding: "Everything that has gone on before this season is on Luther's brow when you see him.
"In this season we find him dealing with that and being the best detective he can. We don't overly explore his personal life until we have to."
He continued: "This series we have one case across four episodes and it gives the writer Neil Cross an opportunity to zero in on what makes Luther good.
"The crimes in this season are particularly gruesome and very well thought out.
"Because we concentrate one character, one crime throughout the four episodes, we get into detail. But also over the four episodes, there are other storylines that start to show up.
"I definitely feel that this season is dark."
Luther returns to BBC One on January 1 at 9.00pm.
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