Watch: Kelly Macdonald compares Operation Mincemeat to Line of Duty
The 46-year-old told Yahoo that her guest lead role in the blockbuster BBC crime series was "challenging but very rewarding", despite the intense pressure.
Operation Mincemeat sees Macdonald's character Jean play a key part in a stranger-than-fiction Allied plan to fool the Germans and conceal their plans to attack Sicily.
It's based on the true story of an elaborate ruse in which British forces used a corpse with a false identity to convince the Axis forces to divert military numbers away from Italy.
Much of the film's action takes place in the covert team's basement operations room, which created a pressure cooker atmosphere for the actors on set as well as their characters.
Macdonald said: "I'm much happier in an ensemble where everybody is in the room doing their own work. It was a great set and we were in there for quite a while. I think the art direction was incredible.
"But Line of Duty was bright lights and stark and it's just talking. It was challenging, but very rewarding."
In the sixth series of Jed Mercurio's show, Macdonald played DCI Joanne Davidson, who came under the suspicion of anti-corruption unit AC-12.
She received acclaim for her performance, including in a lengthy and revelation-packed interrogation scene during the penultimate episode of the series.
In Operation Mincemeat, Macdonald's Jean is a figure of affection for both key architects of the plan — likeable flight lieutenant Charles Cholmondeley (Matthew Macfadyen) and married intelligence officer Ewen Montagu (Colin Firth).
Jean and Montagu bond as they work out the details of their fake airman's backstory and this seems to push them into romance, despite Montagu's status as a married man.
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Macdonald describes the chemistry between the duo as "a very innocent love affair" informed by the excitement of helping to build a deception together.
She said: "The lines [between reality and their deception] are blurred slightly and I think it's depicted really sweetly in the film actually. It doesn't go too far.
"If it was a Hollywood film, it might be something else entirely. It was a great part to play. I had basically two Mr Darcys."
Operation Mincemeat also stars Johnny Flynn as James Bond author Ian Fleming and Jason Isaacs as Admiral John Godfrey — believed to be Fleming's inspiration for spy boss M.
The movie is directed by Shakespeare in Love's John Madden from a script by Michelle Ashford, adapted from historian Ben Macintyre's non-fiction book.
"We were the little moles in a basement scurrying around and coming up with outrageous plans," Macdonald said.
Operation Mincemeat opens in cinemas in the UK from 15 April.
Watch: Trailer for Operation Mincemeat