"The backpack stays with him," declares Sanjeev Bhaskar. "You never saw Columbo without his raincoat. You never saw McCloud without his horse. You never saw Kojak with hair. These things are sort of keyed in to the DNA."
It may have seemed a minor wardrobe decision when Bhaskar insisted on wearing a backpack to play DI Sunil "Sunny" Khan in Unforgotten. But the hefty black bag, worn nonchalantly over one shoulder at all times, has become a viral phenomenon in its own right.
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"I don’t know if credit is the right word – fault, perhaps," the actor says, explaining how Sunny's backpack came to be. "But they’d said at the beginning, ‘We’ve got some briefcases and things for you,' and I said, 'What about a rucksack or a backpack or something?' – and they said ‘Oh, why?’
"And I said it was because it’s practical. He’s got kids, so he’d have to cart their stuff around and it’s not like you have a bag with ‘POLICE’ written on it. You know, you carry your stuff in it and it struck me as something that is really practical that you could put latex gloves and swabs, that sort of stuff in there, but you could also put in an extra jumper."
Bhaskar made a persuasive case (who doesn't need a spare jumper?) and won the right to wear the backpack on screen – but even he was surprised at how viewers latched on when the first series went out in 2015.
Taking a break from filming series three's police station scenes at an old mansion in Buckinghamshire, the actor – temporarily separated from his backpack – tells us: "People understandably fixate on things that they are particularly interested in, so you know, I’d presume that people involved with police would look at the police aspect of it, they’d be drawn to that.
"Who is drawn to the backpack thing? There’s obviously some sort of backpack fetish."
He adds: "Particularly on social media, there was this thing, ‘What’s with the backpack?’ and I’m thinking, ‘It’s just a bag where you carry around stuff, isn’t it?’"
But even if he has been bemused by the reaction, Bhaskar has also indulged viewers' curiosity about the bag's contents ever since he returned for series two.
"I said to costume and props, “Why don’t you just fill it with stuff which is a surprise to me? Don’t tell me what it is. Just fill it because it’s got to look like it’s got stuff in it'," he says.
Then as a special treat for the backpack fetishists, he'd give them a sneak peek inside. The contents have been... interesting.
"What they then decided to fill it with was really good fun," he explains "I’d carry it around and I couple of times I’d go, ‘This is really heavy this week’ and then at the end of the day, I’d take a picture of it and yeah, it’s weird how that became a thing. I just thought it was a very, very practical decision on my part."
The backpack has become such a star that there has even been speculation it could have its own show.
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"Someone did suggest to me that that could be the spin-off," says Bhaskar. "If I was a ‘70s cop, I could be a backpacking detective."
He adds: "Be a bit like Mary Poppins’ bag, they’ll go, ‘Has anyone got a bomb?’ and I’ll go ‘Yep’. ‘Has anybody got a coffee machine?’ ‘Funnily enough, I’ve got one here...'"
Unforgotten series three will air from Sunday 15th July at 9pm on ITV