Line of Duty's Kelly Macdonald planned to turn down series, until Keeley Hawes stopped her

But Kelly Macdonald, who stars opposite Vicky McClure, Adrian Dunbar and Martin Compston as this year's antagonist DCI Joanne Davidson, says she had second thoughts about taking on the role.

It was actually Keeley Hawes, who won plaudits for her portrayal of DI Lindsay Denton in series two, who convinced Macdonald to stick with the series.

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Macdonald explains all in the new issue of Radio Times. "The real issue was the sheer number of words I had to learn," she says.

"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."

Macdonald, who also admits she'd never seen the show before she was cast in it, other than the odd clip on Gogglebox, says co-star and fellow Scot, Compston helped to reassure her when she saw him at an awards do.

"He was so excited. He said, ‘Hey! You’ve got to do it!’ It got to the point where I didn’t feel I could run away from it. I thought it might actually be a good challenge and it really was. Although I still can’t believe I got through some of it."

We haven't got long to wait now, until we meet Macdonald's character and discover how she will fit into the plot, but she teases that Jo has a "steeliness" about her. "When she walks into a room, everyone is deferential and calls her ‘Ma’am’, which I found quite funny. I struggled with having that kind of authority!”

To read the full interview with Kelly Macdonald, pick up this week's issue of Radio Times magazine, out 16th March. The sixth series of Line of Duty begins on Sunday night, BBC One at 9pm. To find out what's on telly tonight, check out our TV Guide.