The actor speaks in an English accent when he is playing Detective Sergeant Steve Arnott in the BBC Two crime drama - and even keeps up Arnott’s tones off-set.
He told Radio Times of shooting the interviews in the glass office: “It’s a draining couple of days’ filming. You do maybe 20 takes of up to 25 pages of script on the first day.
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“Making series two, it was the height of the summer - it was very, very hot and the lights made things worse. People were drifting off - it was hard to keep concentrating.
“Fortunately the new boxes for series three are bigger and better ventilated. You still sweat, though, and you get so tired. Sometimes I hear my accent slipping and the Scots burr leap in.”
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Compston, 31, said in series one they considered shooting the scene in different takes, but Lennie James (who played DCI Tony Gates) wanted to run the full scene.
“He was right - it changes the scene completely when you do it through in one take,” he said.