Martin Freeman was so determined to give a believable performance as a Liverpudlian that he spent the duration of his stay in the city speaking with a Scouse accent.
The TV and film star stars in The Responder, an original BBC drama, which airs on Monday (January 24).
The six-part series was written by ex-Merseyside police officer Tony Schumacher.
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For the duration of its filming - in locations such as Everton, St Helens and Wirral - Martin spoke with a Liverpudlian dialect, including after the cameras had stopped rolling.
The ECHO spoke with Tony, who left Merseyside Police in 2006, ahead of the series' air date.
He said: "Because of covid I wasn't actually on location with Martin much, but he rang me all the time. I'd be out and he'd ring me saying things like 'Eh Tony, how do you say cake?'
"He was determined to get it right, and because of that he stayed in the accent the whole time, even when we weren't filming.
"When lockdown lifted and you could do more, go to cafes and stuff, Martin loved it because he could pop a mask on and go to these places where normally he couldn't. And his Scouse accent helped."
Tony said Martin would pop to local beauty spots for walks, and head to coffee shops, staying under the radar with his new accent.
He continued: "It was great for him because he could go and have a brew and [between the face mask and the accent] nobody would know it was him.
"You know if you've managed to get past the old dears in cafes on Smithdown Road, you've nailed it."
When filming had ended, the cast and crew went out to celebrate, and it was then that Martin "became a cockney again".
Tony added: "I'd actually got to the point where I forgot that was his real voice.
"Honestly, within five minutes of watching, you'll have forgotten he isn't a Scouser."
The Responder airs from Monday at 9pm on BBC One and on BBC iPlayer.
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