Friends for 20 years, the pair say they had been discussing their idea for a buddy cop drama for a while before it was eventually made.
The pair play maverick detective inspectors Aaron Bishop (Clarke) and Ronnie Pike (Walters) and say they were keen to show positive representation of two black male leads on screen, saying they were "sick" of seeing the same old characters.
"Shows, predominately shows with people like myself and Ashley, especially two men, don't always get made or we just don't see them," Clarke tells Sky News. "That's what makes Bulletproof so special.
"That's why everyone says 'we haven't seen a show like it'. Well, duh. Because shows like it don't get made. And that's why we're kind of the first people to do that. And that's what makes it special - one of the things that makes this special."
Released in 2018, the first series introduced viewers to Bishop and Pike, balancing gritty action with comic moments and family moments, as they chased down hardened criminals through London.
Hugely popular with viewers, it became one of Sky's biggest dramas that year and is now due to return for series two.
As co-creators who had fought for the show for so long, the reaction felt "amazing", says Walters.
"I'm enjoying it still now," he says. "It was brilliant for us. It meant that all the hard work that we put in before, the fights that we'd had to kind of get it to where it was, just paid off and it all made sense.
"We always knew that there was a place in people's hearts for this show, there was a gap in the market for this show, but we didn't... we never knew how great it would be and we're overwhelmed, we're honoured."
Actor, screenwriter, director, BAFTA and Olivier award winner, Clarke, 44, made his name with his landmark film Kidulthood, which he wrote and starred in, in 2006, and its sequels Adulthood and Brotherhood, as well as shows including Doctor Who.
Walters, 37, is best known for roles in Bullet Boy and Top Boy, as well as Bulletproof and his music work; he now performs as Asher D, but rose to fame as a member of chart-topping UK garage collective So Solid Crew in the early 2000s.
Outside of their characters and show, the pair are good friends. The on-screen chemistry is genuine.
"There isn't a week that goes by where we don't communicate," says Clarke.
"You don't understand how funny this show is," says Walters. "It's not a comedy, it's an action drama with some laughs in it. But it feels like you're making a comedy when we're on set."
"Sometimes, occasionally, you know, we'll even laugh so much about something that we'll go, we need to put that in, and we'll try and find a way to get it in, in the show," says Clarke. "And a few times we've succeeded."
Which is perhaps how they managed to squeeze in a surprise nod to Walters' So Solid Crew past in the opening episode of the new series.
"Yeah, they kept that in, I was surprised," he laughs. "I'm not sure how I feel about it but yeah, it's there."
He adds: "This is what happens when I'm filming by myself... and everyone else has gone home. What happens sometimes is we'll have, like, five scenes in a day and then there'll be one on the end that's me, just by myself. And I've got to stay longer than everyone else.
"That was probably one of those days, where... it's just me and the cameraman. And I probably did about 55 different things and they pick that one."
Clarke says he was sceptical at first but is now enthusiastic.
"It's going to work," he says. "It definitely will work. The keyboards are gonna light up after that moment. Definitely."
Clarke comes across as something of a perfectionist. I get the impression he thinks a lot about how lines will play out, how viewers will react.
"Yeah, I definitely think about it," he says. "I think about things like that way before [filming]."
Walters, he says, is his comedy gauge. "I'll tell him what I'm thinking; if he laughs, I'm like, yeah, okay. Yeah, that's good."
Most of the research for Bulletproof, working with real police officers, came ahead of the first series, when they underwent firearms training and got to hear about undercover work.
"They go through a lot of stuff," says Clarke, speaking about undercover officers. "There's probably things they go through that if you put [them] on screen, people would think were exaggerated.
"They get deep. There's stories of people having babies, deep undercover stuff. None of our characters go that deep but it was really a little bit of an insight that anything that we do on the show, even if it feels a bit heightened, it's probably happened [in real life]."
"[There was] this woman that had gone that deep undercover that she had two kids with a guy," says Walters.
"With the gangster," Clarke adds. "I don't know. I guess there's kind of got to be a bit of Stockholm syndrome or feelings for the person. But definitely when it was time to do the job, she still did the job, sent him down or whatever. And you've got two kids with him. That's crazy."
Luckily, Clarke and Walters can leave the police work behind them when they step off set. Both agree they probably wouldn't be cut out for it, although Clarke says he is comfortable "making tough decisions".
As actors, how do they feel knowing their work, portraying successful police officers on screen, is helping other aspiring actors from different backgrounds?
When they were younger, the idea of carving out a successful career on screen was an impossibility, says Clarke.
"I think we are helping by the very nature of them seeing what we do," he says. "Ashley's younger than me but started probably before me - when we started out it was an impossibility. Not just an improbability, but it was an impossibility.
"The success that some of these guys are having now, this just couldn't happen. So I think by the very nature of the stuff that we've done over the last 20 years - and it has been 20 years since we met - people have seen that and it's opened doors for a lot of people behind us, you know. So I think it definitely is important for us to do that."
However, Clarke says he doesn't see himself as a role model. Gesturing to Walters, he says: "He has got kids, I've got kids.
"It's not our duty to do it for people. My duty is to look after my children, his duty is to look after his children.
"But I think by the very definition of what we do and the positive things we've done, and the doors we've opened, people could see that and look at that, look at examples that we've set, and follow or go through the open door, or take the ball further than we've taken it, because we took it to a certain point. So I'm proud to be one of those people.
"You know, someone said to me the other day at an event, 'you laid the foundations'. And part of me's like, yeah, I did. You know?
"I also had to remind him, I'm not dead, bro. I'm still here. You know, he said to me, 'you blazed a trail, now I'm on the tracks'. I said, listen, I'm still here. I'm at the same event you're at.
"I feel like it's important we've done that. But I don't think it's our duty to necessarily be like, 'follow me, I'm a role model' - because I'm definitely not that."
He might not consider himself a role model, but Clarke is appreciative of his fans and what he and Walters mean to them.
"People that support you are the bread and butter. Those are the people that watch the show, the people that buy a DVD of our films, those are the people that subscribe…
"It's people who are going to be that polite and go out of their way and fumble to take a picture and get excited. I think it's part of our job that we make sure we facilitate that.
"It kind of doesn't matter what day I'm having. If it makes them happy, I need to do that."
But both agree that they don't consider themselves famous.
"Tom Cruise is famous," says Clarke. "Brad Pitt is famous. Will Smith, Denzel [Washington], they're famous."
"Fame is... for me, anyway, is where it stops you from having a life," says Walters. "But I can. I'm having a life right now.
"I get on the tube, I get on the bus... we're pretty much still cool to do our thing."
As two actors who through their work have raised awareness of the need for people of colour and from different backgrounds on screen, I ask about diversity and what more needs to be done.
Clarke turns the question back on me. "What do you think?"
I tell him I think changes are being made, slowly, but there's still a way to go. We're still a way off balanced representation. But I'm not in the industry - I want to know his experiences.
"There you go," he says. "Equality is when people are equal, right. When sexes are equal. So I think until those things happen there's always work to be done, whether you're a woman, a person of colour or not able-bodied, you know, equality is the definition that all people are equal.
"Sometimes it's difficult because when you have a vast majority of one set of people, like able-bodied people, they're not always thinking about the people that aren't able-bodied. So you have to kind of change your mind-set and get into that.
"Things are getting better, but they're not quite there yet. But I'm glad to be part of it, one of the people that's helping change things."
Series two of Sky original series Bulletproof will air on on Sky One and NOW TV on 20 March