IN a London hotel room, a slightly jet-lagged Mark Ruffalo is talking about his childhood. It's a story of comic books and Catholic schools; just the kind of stuff you might expect of someone growing up in Wisconsin in the 1970s. Wisconsin or Wishaw for that matter. Normal. Happy, the usual.

But then he starts talking about the priests. The priests you would stay away from. "There was always a priest that was a little touchy and the kids – except for the kids on the margins – stayed away from those guys. Something about them that didn't feel right. You didn't know what it was but you just sensed it, you know?"

Ruffalo has made a film about those priests, or ones worse than that. In Spotlight, the movie he's in London to promote, Ruffalo plays a member of a team of investigative journalists on the Boston Globe back at the turn of the century who revealed the extent of clerical abuse in the city and the lengths to which the Catholic Church, then under the leadership of Cardinal Bernard Francis Law, went to in order to cover it up. So as someone who grew up in the Catholic school system (though his dad was in fact Ba'hai) the obvious question to ask Ruffalo is whether, growing up, he was aware of any cases of abuse.

The short answer is no. "In my immediate school and in my immediate experience there really wasn't that kind of thing. There were other kinds of abusive behaviour. Physical punishment that today would not be accepted, and verbal abuse. Things that fly in the face of what we imagine Christ was espousing when he was talking about love and tolerance."

But if the greater crimes didn't go on in his world it went on elsewhere. That truth is at the heart of Spotlight. Stories of priests abusing the children in their congregations. Stories of priests being moved from parish to parish as a result. Stories of church cover-ups. "It was shocking," Ruffalo says. And it needed to be talked about. "People make jokes about priests being paedophiles all the time. That doesn't just come from out of the blue. That comes from a cultural need to talk about something that's taboo."

Ruffalo is now part of the conversation, of course, with Spotlight. He's in the UK to promote the film in which he plays Mike Rezendez, one of the reporters who broke the story. It's an ensemble piece, but Ruffalo-as-Rezendez is all tousled single-mindedness and on-the-verge-of tears speechifying. It's a role – and a performance – machine-tooled to get attention. He's duly been Oscar-nominated as a result.

But then Spotlight is a fine example of old-fashioned Hollywood craft. Showbiz liberalism, in which our heroes (journalists, who'd have thought?) take on the might of the system, in this case the Catholic Church. It's old-fashioned in its unflashy storytelling, its lack of special effects and its idea of cinema as a crusade.

Old-fashioned too in its vision of investigative journalism. Some 15 years on from the film's setting it's difficult to imagine journalists being given the time and the resources to investigate the story in the newspaper industry's current scorched earth environment. "That was apparent going into it," admits Ruffalo, "and it became more acute. There's definitely been a shift in investigative journalism and I think what this story does a good job of is reminding us of the importance of this kind of work.

"We often get to see stories of journalism going badly in films. We rarely get to see journalism working at its highest level."

Spotlight has notched up Ruffalo's third Oscar nomination. Third time lucky perhaps. He was previously nominated for Lisa Cholodenko's spry 2011 comedy The Kids Are All Right and another biopic, Foxcatcher, last year. The latter and Spotlight required what he describes as "due diligence" in terms of his preparation. "Some movies I just daydream about and don't have to do much 'investigative journalism'", he says. Not so here. He tagged along with Rezendes, did his homework. It matters, he says. "There's too much on the line to play fast and loose. This particular story – or Foxcatcher – I felt you really had to get it right because you're talking about people's legacies. You're talking about their credibility. You're talking about their careers. It's important."

It is the kind of role – the kind of film, for that matter – that is designed to catch the eye of Academy Awards voters. But Ruffalo has been catching the eye in films since the late 1990s. His screen appeal is almost eponymous. He does have a rugged charm and a slightly ruffled manner, with a side order of edge if required. One writer described him as "the wounded hipster neurotic of modern cinema".

Maybe the wounds are real enough. In 2002, just a couple of years after a breakthrough role opposite Laura Linney in You Can Count On Me, and not long after the birth of his first child, he was diagnosed with a brain tumour. It turned out to be non-malignant but the operation to remove it paralysed one side of his face for a time.

He thought about giving up acting at that point but a role in Jane Campion's flawed erotic thriller In The Cut, a bruised black flower of a film in which he plays a policeman who might be a murderer, kickstarted his career again. And then in 2008 his brother Scott was murdered in Los Angeles. In the aftermath of that trauma he somehow managed to direct his first movie Sympathy For Delicious and believed maybe he'd gone as far as he could as an actor.

And yet here he is. Why is that? "I'm an actor. That's it," he says. "There was a moment when I was accidentally mistaking acting for the business of acting and they're really two different things. That was probably after my brain tumour. And then just before The Kids Are All Right I really felt maybe I'd run the course on acting. It got muddled. And some of the people around me, their control over my career, made me uncomfortable."

Directing made him rethink what he did, freed him up even, and so when he came to make The Kids Are All Right he remembered what he'd loved about acting in the first place.

It was noticed. In the last few years there have been the aforementioned Oscar nominations and even the chance to play one of his childhood favourites. In the 2012 Marvel film The Avengers the comic book reader that he was got the chance to play a comic book hero. As the Hulk he manages to bring a touch of humanity to the CGI excess. "And by the way, the Hulk was my guy so the fact that I'm playing him is another dream come true," he tells me, his voice lightening with delight.

"It's a great gig. It's like having a television show where you shoot one episode every three years and you get paid the same amount as if you'd been working every day of those three years. And my kids love it and people love it and it feeds into a really interesting mythology that's just fascinating to me."

But on its own, you imagine, it wouldn't be enough. As well as being a husband and a father, Ruffalo, now in his late 40s, is an outspoken liberal activist. He is a visible and vocal opponent of fracking. And on his Tumblr he's backing Bernie Sanders for president.

He's happy to trace some of the origins of that activism back to the morality he picked up in Catholic school. "I took an enormous amount of my world view from those teachings and later my father's Ba'hai faith."

How then does Ruffalo view things in his own back yard? What's his take on the controversy surrounding this year's none-more-white Oscar nominations. "I understand it," he says when I raise the angry response to this year's list. "I'm very sympathetic to this burgeoning movement of righteousness that's coming from the African-American community in America predominantly with the Black Lives Matter movement. The way that community has dealt with the institutionalised racism that's prevalent in many of America's institutions is not terribly unlike the Spotlight story in a way. You have institutions that have been looking the other way, so to speak. It's not new. We're just seeing it in a new light."

The idea of boycotting the ceremony had been considered. However on Twitter the other week he tweeted: "To clear up any confusion. I will be going to the Oscars in support of the victims of clergy Sexual Abuse and good journalism." He's glad that at least things haven't been swept under the carpet.

"Listen, I'm a white guy who's been nominated and so it could be a very touchy subject but I completely understand it and I think it's appropriate and I think it's a discussion we should be having."

The same could be said of the clerical abuse saga. Does he think the Catholic Church done enough? "No, I don't. Two incredible stories have come out in the past week or so. One of them was the 200 kids who were [allegedly] molested or physically abused in the Regensburger Domspatzen choir in Germany [the number may be as high as 231 in fact] and then the Seattle archdiocese just published a list of priests who have been accused of paedophilia that they've been sitting on. But there are archdioceses around the world who are still covering up. More transparency is definitely needed."

And then he points to Cardinal Law, the leader of the church in Boston who presided over the events covered in Spotlight. "The fact that Cardinal Law is sitting in a palace in the Vatican with an annual stipend does not carry any kind of justice over what he presided over."

Spotlight is in cinemas now.