Arnott’s love of three-piece suits was actually inspired by a friend of Compston, who told the actor about a smarmy colleague who – you guessed it – made a habit of always wearing waistcoats to work.
“Before we started the first series a pal of mine who works in a call centre was telling me about someone who worked with him,” he told the Evening Standard.
“This guy was a right wee dick and he wore waistcoats to work — who would wear waistcoats to work in a call centre?”
But – unexpectedly – Waistcoat Man was popular with the ladies, “which nobody could understand because nobody liked him. But it was the waistcoat that did it, seemingly.
“And I thought: ‘That’s him – that’s Arnott, the needlessly overdressed guy.’”
Even Compston describes Arnott as an “arrogant wee prick”. But with his seductive waistcoat, could he be after Vicky McClure’s DS Kate Fleming?
“We do laugh about this,” he says. “I think it’s more of a brother/sister thing.
“There is something more to their relationship because they do drive each other mad. But we’ve had the conversation and we’re not Mulder and Scully, and you’d ruin something by going there. And also I think Kate knows Steve’s a wee f***er – she wouldn’t fall for it.”
The Line of Duty series finale airs on Sunday 30th April at 9pm on BBC1