Neil Maskell hasn’t got a bad word to say about anyone. His Kill List co-star Michael Smiley is “like family to me”. Iain McDiarmid is “amazing”. Simon McBurney is “a great genius”. Even Danny Dyer, with whom he did his first theatre work aged 18, is “a lovely man who gets a hard time and doesn’t deserve it”.

A more genial actor you could not hope to meet. Until, that is, we start talking about one particular scene in the first series of Dennis Kelly’s lurid, paranoid thriller series Utopia, when Maskell’s character Arby, a wheezing assassin in a grey pleather jacket, wanders into a school and calmly murders six children and two teachers. Despite it being broadcast just a few weeks after the Sandy Hook massacre in Connecticut, it somehow managed to avoid a frothing tabloid furore, though on a different day of the week, it might not have been so lucky. Ofcom received 37 complaints about the scene, Channel 4 a further 28.

“People complain about the Antiques Roadshow,” growls Maskell, sitting forward and getting gradually more irate. In doing so he displays more than a hint of the on-screen menace that he seems able to turn on whenever his roles demand it. One minute he’s all smiles, the next you wonder whether he might just flip the table. He doesn’t, fortunately.

“Of all the things in this world to shout about, to write a letter about... if it’s something you’ve just watched on the telly, you need to have a right word with yourself,” he says, getting into a rhythm. “I find that offensive. If you’re going to get a petition going, make it about something that’s actually affecting and damaging people’s lives. I guess what I might find jarring, or more offensive, is our government’s complicity in horrors that go on on a daily basis, than some Channel 4 drama.”

So he didn’t find the Utopia scene gratuitous? “I thought it needed to go that far. [Arby] is effectively a child soldier. We accept that that is going on in some countries. It’s not something we’re doing a lot to change. Kids kill kids. And if that was the point, then we have to do it, or you compromise the whole thing. You soften it. And I think that’s worse.”

Utopia, which returns for its second series this week, boasted more than a few water-cooler moments of extreme violence. But it was also notable for the calibre of its cast. Veterans such as Stephen Rea, James Fox, Simon McBurney and Geraldine James shared screen time with comparative newcomers Alexandra Roach, Fiona O’Shaughnessy and Nathan Stewart-Jarrett. One night after filming, Maskell found himself drinking cocktails and talking with Fox about his role in seminal 1970 drama Performance, and then the following night over dinner, grilling Rea about Neil Jordan and his career in political theatre in Belfast during the 70s. “That’s the reward,” he says, eyes shining. “To get to spend time in the company of people you admire. And to learn. That’s the juice.”

Maskell had wanted to be an actor since primary school. Raised in Erith, south-east London, a friend of his dad went out with the actor Cindy O’Callaghan. She recommended the Anna Scher Theatre school in Islington, where Kathy Burke and Phil Daniels studied. He started classes in 1987, aged 11.

“Secondary school was a bit of a hateful time for me. I wasn’t a bad lad, just disinterested,” he says. “It felt like an antiquated way of teaching and discipline, like it hadn’t moved on much since Victorian times. [At Scher’s] we had a common interest. It was much more exciting than anything going on in suburbia.”

Taking the classic Brit actor rite-of-passage route, the school-age Maskell popped up in the likes of The Bill, Casualty, Soldier Soldier and London’s Burning. Then Gary Oldman came by Scher’s when Maskell was 18 and gave him the part of Schmuddie in Nil By Mouth. That occasioned more film work: football fans still mob him in the pub thanks to his role in Nick Love’s oft-maligned The Football Factory in 2004. But being a jobbing actor has had its ups and downs. One year, he recalls, he had just two auditions.“I always had to do other things, until my 30s, really,” he says. “Carpet fitting, gardening for the council, warehouse work. But people have harder lives. I had this extra ‘acting lottery’-type thing, where any day the phone could go, and you’d be up for something. I haven’t had it hard, I’ve had it bloody easy, actually.”

It was likely this grounding that kept his expectations in check. “I was always fairly realistic about the fact that I’m not a matinee idol,” he says. “I’m not going to be George Clooney, let’s be honest. I’ve set my ambitions fairly low, and I know actors better than me who have had to leave the business. Everything’s turned out far beyond how I thought it was going to, and I’ve got Ben Wheatley to thank for that.”

It was on short-lived BBC3 sketch show The Wrong Door that Maskell met Wheatley. Maskell had no idea, but over two days of mostly improvised comedy, he’d made a significant impression on the director. A couple of years later, Wheatley shoe-horned Maskell into a viral ad he was making for Heineken in Romania, and while sharing a Jacuzzi one morning, as one does, Wheatley revealed that he planned to cast him as the lead in a black comedy caper set in the Philippines he was thinking of calling Get Jakarta. That film didn’t happen, but parts of it became the basis of jarring horror Kill List, in which Maskell played a contract killer haunted by a past job in Kiev, who becomes encircled by a murky underground society with distinctly pagan leanings. It was a role written specifically for him.

“He’s just naturally interesting to watch,” says Wheatley. “He seems incredibly scary on screen, but actually he’s a lovely person to be around. It must be a weird one for him. I expect he gets all sorts of stares in the street. But I can see him doing really big parts until they have to pension him off.”

In the space of a few roles, then, Maskell has upped his stock dramatically, from football thugs and east London bad lads to something more engaging. He’s shown his aptitude for comedy, too, in the unexpectedly heart-warming Channel 4 series The Mimic, returning this week. But Maskell admits that returning to play Arby in Utopia a year on proved challenging. While he’s reticent to mention it for fear of looking pretentious, he says that Velázquez’s paintings of courtly buffoons helped him find a new line on the character, portraying a simple-minded man with dignity and respect. “I found it very stressful,” he admits. “I was so worried that what I was doing was an impersonation of myself! I got there, but there were some creative struggles. In a good way.”

His most recent project, Hyena – a grimy take on police corruption –– opened the Edinburgh film festival last month. He’s also made some shorts, and hopes to spend more time behind the camera. But those years as a jobbing actor have kept Maskell anchored. “I’d like to do a film, a couple of good tellies and a play a year. Maybe direct something,” vhe says. “And that’s it. It’s a modest ambition.”

Utopia starts Mon, 10pm, Channel 4; The Mimic starts Wed, 10pm, Channel 4