Kiwi audiences already know and admire Viggo Mortensen for his towering portrayal as Aragorn in the Lord of the Rings trilogy but now, with Green Book, there's a whole new side - and size – to him. That's because he had to pile on the pounds to portray Tony "the lip" Vallelonga, a wisecracking Italian-American bouncer who's hired to chauffeur – and guard – Dr Don Shirley (Mahershala Ali), a renowned, black pianist, during his groundbreaking road tour of America's Deep South, in the 1960s.

"Getting there took a bit of time but it wasn't too hard to gain 20 to30lbs [9-14kg] - I just ate a lot of fried chicken," says Mortensen, explaining his weight-gain regime. "It was much harder to lose the weight though. I remember Mahershala asked me how I was doing, after filming, and my reply was, 'I'm still as fat as a tick, in June', because I just couldn't get the weight off."

It's not the only notable transformation associated with Green Book. Director Peter Farrelly, best known for his off-the-wall, often-offensive comedies, like King Pin, There's Something About Mary and Dumb and Dumber, underwent a seismic shift too: he ditched his trademark bad-taste, goofball yarns for a poignant, thought-provoking, comic-drama. It's a change that's just nabbed him his one and only award, a Golden Globe for Best Screenplay.'

"I think people who know Pete's work will be very pleasantly surprised that this is one of his movies," says Mortensen. "I think they'll be moved by it too, because I know we've made a really good movie. The intensity of people's reaction to it at screenings has been amazing. It's been much more vocal and passionate than I expected."

Dr. Donald Shirley (Mahershala Ali, right) gives an impromptu performance in Green Book. Photo / supplied
Dr. Donald Shirley (Mahershala Ali, right) gives an impromptu performance in Green Book. Photo / supplied

It's easy to see why, with Green Book telling the unlikely true tale of how Don Shirley and Tony Vallelonga's two-month road trip across the States transformed the odd couple into firm friends while highlighting the bigotry and racism of 60s America. At its core, it's a heart-warming bromance that lasted right to the end of their lives, despite their vastly contrasting backgrounds and upbringing.

"It is remarkable how they became such good friends, especially as at the start they seem like they're worlds apart," says Mortensen. "But they're blinded by their first impressions of each other, so when Doc looks at Tony and sizes him up he thinks he's ignorant, rude and of limited intelligence. Similarly, Tony's first impression of Doc is he's eccentric, a snob and emotionally closed-off.

"But those first impressions are so wrong, and that's what they find out over time. I think we can all learn something from that," Mortensen's says. "We should all try to not go with our first impression of somebody, or judge a book by its cover. Instead, we should take the time to get to know someone without judging how they look, what they sound like or where they come from because not doing that is prejudiced, uninformed and ignorant."

From left: Viggo Mortensen, Peter Farrelly, Linda Cardellini and Mahershala Ali with their Golden Globes for Green Book. Photo / AP
From left: Viggo Mortensen, Peter Farrelly, Linda Cardellini and Mahershala Ali with their Golden Globes for Green Book. Photo / AP

Aside from the moral messages underpinning Green Book, it's Mortensen and Ali's pithy exchanges, on-screen banter and charismatic portrayals of Tony's brazen vulgarity and Don's paradoxical gentle refinement that make their friendship and relationship so compelling. It's a portrayal that's earned Ali a well-deserved Golden Globe for Best Supporting Actor, to add to Farrelly's Best Screenplay award.

"I think you can't but help root for these guys, and their chemistry – or lack of it," says Mortensen. "But seriously, despite their differences, they're both likeable in their own way. That's really important, as is the audience believing in them. That's why my acting is only as good as my reacting. It's how I react to Mahershala, it's how he listens to me. That's what makes it funny. That's what creates the humour."

Equally important is how believable and authentic Green Book is. To ensure its accuracy, the screenplay was based on a story written by Tony Vallelonga's son, Nick, who had listened to all his dad's tales as a kid, written them down and squirreled them away for posterity.

"Obviously, Nick knew everything about this story, so he was really helpful to us, in that regard, as a reference," says Mortensen. "And then for me, personally, having him there to help me with certain turns of phrase, or just the way his dad sounded and behaved was invaluable.

"Actually, one of the funny things about shooting this movie was that the screenwriters were there every single day. That's really unusual on a film because usually the directors and producers don't want them there," he explains. "But on this film, everyone was pulling together to make the script and the movie the best we could. So normally the three producers and the scriptwriters were there every day. Mostly that was great, but sometimes they were like the Three Stooges, squabbling and being ridiculous together. That was quite a thing to see."