Kelly is set to appear as new antagonist DCI Joanne Davidson in the sixth series of Jed Mercurio’s hit BBC drama, who is described as the “most enigmatic adversary AC-12 has ever faced”.
However, the Scottish actor almost “ran a mile” after seeing the scripts, and turned to Keeley – who appeared as corrupt officer DI Lindsay Denton in series two and three – for advice.
Kelly, who had not seen Line Of Duty prior to being offered the role of DCI Davidson, told Radio Times: “It’s a horrible thing to ask of someone! But the real issue was the sheer number of words I had to learn. When I saw the script, my first instinct was to run a mile. It’s all addresses and dates and police jargon, especially when I was interviewing suspects.
“Keeley was brilliant; she talked me down from the ledge. She said it might look as though you are being asked to do something completely impossible, but it’s not.”
After accepting the role, Kelly then attempted to get herself up to speed, having only seen bits of the show on Gogglebox previously.
She said: “I got halfway through season four and stopped because it was getting too close to the start of the shoot. I was getting proper nerves and watching Thandie Newton [who played DCI Roz Huntley] wasn’t making it easier!”
Kelly’s character is leading the investigation into the murder of journalist Gail Vella, with AC-12 tasked with establishing whether police corruption is the reason the case has gone unsolved.
Adrian Dunbar, who plays Superintendent Ted Hastings, previously revealed the first episode would be a “head wreck” for viewers as the action has moved on 18 months since the last series and many things have changed.
Line Of Duty returns on Sunday 21 March at 9pm on BBC One, with the new season including an additional episode, with seven instalments instead of the usual six.
The new issue of Radio Times is on sale now.