There was a reason America- born actor William Sylvester was available for an American director who only filmed in Europe and lived in England since both Sylvester and Stanley Kubrick ala 2001: A SPACE ODYSSEY were British staples: In fact the former was in as many British b-movies as any English-born b-movie actor...
Some good, others okay, OFFBEAT aka THE DEVIL INSIDE provides the usually smug, deliberately cool/calm performer real edge and intensity, starting out wearing dark sunglasses while robbing a bank... then meeting with fellow cops for whom he's an undercover agent...
OFFBEAT takes place mostly during one heist, and it's a tunnel job on a quiet city street...
Sylvester's job is to infiltrate a syndicate of big-time thieves; yet this den is full of working-class types so crooked they're almost normal, including by-the-numbers leader Anthony Dawson and affable John Meillon, who, along with some muscle, pose as construction workers.
But it's the gang's token ingenue Mai Zetterling that matters, lovely and tough despite falling too hard too fast for our anti-hero, constantly dogged by his Scotland Yard boss... which, as he and we learn, could be for understandable reasons, and is thus Sylvester's best crime-flick role, juggling a kind of criminal/cop ambiguity like only a good Noir can.