You’ve got to be pretty brave to invite a film crew into your house. You’ve got to be even braver to agree to your kids visiting prison on a regular basis - and on top of that to commit to five years of filming. But Sarah and Colin Kirk have no regrets about their life-changing decision to welcome prolific film director Michael Winterbottom into their Norfolk semi to make Everyday, his ground-breaking drama about what happens to a family when their father is sent to prison. At the time the Kirks’ youngest child, Katrina, was just three, while Shaun, Robert and Stephanie were four, six and eight respectively.

The film is an ambitious mix of improvised drama and 7 Up-style footage, shot in short bursts over five years, with John Simm and Shirley Henderson playing the fictional parents and the kids just being themselves. That’s not easy when you’re only three and Moaning Myrtle out of Harry Potter is pretending to be your mum and the Master from Doctor Who is your dad.

For a drama about prison Everyday is refreshingly ungritty: its focus is firmly on separation rather than social issues. It’s a theme that will resonate with many families, and not just those with a parent who’s been banged up.

But what makes it unique is that many of the prison scenes were shot in three actual jails (Brixton, Norwich and Stocken), and feature real prisoners and guards.

When the children visited the prison for the first time they had little idea what to expect beyond what they’d seen on TV. Shirley Henderson was equally unprepared. “There was no showing us the prison beforehand; Michael doesn’t want you to be too familiar. He filmed it as we went into the prison for the first time, with the dogs and the warders coming at you. That makes you feel guilty too. The children were probably like me, feeling a bit unsure and thinking, ‘Who are these people?’”

Katrina, now nine, remembers the experience. “I didn’t know what it would be like in prison, but when you walked in it smelt really horrible of dampness and sweat.” Shaun adds, “I thought they’d just ask us questions when we went in, like have you got any drugs. But they started patting us to search us and sent in a sniffer dog.”

“We weren’t allowed to play with the other kids who were visiting their real dads in prison. Going to jail was different from filming at home because we weren’t just pretending, you had to actually go to an actual prison with real prisoners. I was all right with it because I knew the guards would not let them hurt us and would keep us safe.”

The Kirk parents were always there in the background, just out of sight, ready to comfort the children if necessary. When Katrina got upset during one prison visit her real mum had to step into the scene as an extra. As long as Katrina could see her mum she was fine and she soon got used to pretending Shirley was her mum.

Similarly the local primary school playground scenes are totally authentic. “We were told to have a fight,” says Shaun, “but we weren’t allowed to hit properly - so the other people wouldn’t get hurt. Michael didn’t want us to get told off by their parents.”

One advantage to working in real locations over such a long period of time was a growing familiarity between actors and children. They became part of each other’s lives and a natural ease and tenderness developed, which transmits seamlessly to the screen. As a result the kids were quick to pick up on the actors’ moods and come up with highly sensitive emotional responses that made them a joy to work with.

Shirley Henderson has fond memories: “We were very comfortable with each other; we’d cuddle into each other on the sofa watching telly while mum and dad made us cups of tea. And when we were filming the children were more naturally emotional than maybe we’d imagined. You didn’t have to ask them to be upset or happy; they would just offer things up. And that would sometimes dictate what the scene was; a lot of the prison scenes are about the children’s mood.”

For the director the big bonus of the five-year commitment was that it was actually less likely to interfere with the normal lives he was trying so hard to capture. “Often with kids you’re nervous of dragging them out of their normal world and then dropping them back in. Whereas we got to know the family and it’s been a regular thing. We’ve become part of their lives rather than being the weird exception.”

It also meant that there was no need to have children of different ages playing each child as they grow up. And the emotions on the faces of the kids and the actors are real. The story is structured around a series of prison visits, and each time John Simm is genuinely surprised by how much the kids have changed, by their missing teeth and zig-zag haircuts, because they really hadn’t seen each other for a year. And when Shaun bursts into tears because a prison officer tells him off it’s totally spontaneous - although admittedly he then had to replicate the emotion several times to get the shot right.

“That was a real moment,” says John Simm. “There were lots of real moments, and sometimes the kids did get upset. You don’t want to exploit them - you don’t want to make them cry - but sometimes it happened during a scene. When they saw me they had to go through the whole rigmarole of going to prison and being searched, which was quite daunting and scary, but also exciting. And it was all real. There was no trying to learn lines.”

There were times in prison when the fine line between acting and reality became blurred for Simm, too. “I would be swaggering down the prison corridor and I’d hear ‘Get back in your time machine, knobhead!’ and that would bring me back to reality: I’m not a prisoner, I’m an actor.”

The downside of taking so long to make the film was the possibility that the kids might go off the idea, and even refuse to take part when they got older. When asked if he’d let his own kids do something like this Simm is adamant: “No way. It’s like asking me if I’d let a film crew use my house for a week - no chance! But that’s because I’m in the profession.”

And inevitably there were times when the kids tired of shooting scenes several times, especially the older two, Robert, 12, and Stephanie, 14. “Sometimes I didn’t want to do it,” says Stephanie. “It just got really boring - like when we had to walk back to the same spot and do it over and over again. Michael would say ‘Just this last shot!’ and then we’d have to do loads more!” If any of the children said they didn‘t want to take part their dad made it clear it was their decision; but they always came round.

It’s not hard to see what persuaded them, says real mum Sarah, “It was like relatives coming down. The children seemed to just switch off from us and over to Shirley and John. I remember one moment when Shirley and I were in the kitchen making a cup of tea. Robert came running in saying ‘Mum!’ and we both turned round. He said, ‘Not my real mum, I mean Shirley.’”

Everyday is on Channel 4 tonight at 9:00pm.