Since Hamilton debuted in New York in 2015, the hip-hop musical, which transferred to the West End in London this month, has been breaking the conventions of musical theatre with award-winning abandon.

There was the subject matter: the story of America’s least well-known founding father Alexander Hamilton; and the music: a fusion of rap, hip-hop and classic theatrical numbers. And, controversially, there was the diverse cast, with young black, Asian, Latino and mixed-race actors portraying the old white founding fathers and their cohorts.

Timeline How Hamilton the Musical became a smash hit Show

In an interview with the Guardian, the director of Hamilton, Thomas Kail, said there was “no question” the multiracial casting would also be at the core of the UK version, which officially opens on 21 December.

“We never imagined casting the show in any other way – never for one second,” he said. “We are very conscious of what we are doing here. This is not colour-blind casting. It felt essential.”

Kail’s relationship with Hamilton’s celebrated writer, Lin-Manuel Miranda, began in 2005 when he directed Miranda’s debut, the musical In The Heights. He was also one of the first to witness the origins of Hamilton, which began as a single song in 2009. With Kail’s encouragement, the song grew into a 10-track mixtape and eventually spawned an ambitious 46-song musical.

Hamilton might begin in 1776, but Kail said he and Miranda always intended for the show to feel relevant, holding up a mirror to society. In the original production, Miranda, who is of Puerto Rican descent, played Hamilton; in London, Jamael Westman, born to an Irish mother and Jamaican father, takes on the role.

Thomas Kail, the Hamilton director, pictured at the Richard Rodgers theatre in New York City. Photograph: The Washington Post/Getty Images

In the stubbornly white world of British theatre, which remains a long way off from regularly casting black and minority ethnic actors in roles long-played by their white counterparts, Hamilton makes a powerful statement.

“When we started casting this in 2011, we didn’t know the subjects that would be dominating headlines in 2017,” Kail said, when asked whether the simmering racial tensions in America over the past couple of years had made the casting choices feel more pointed.

“What I knew is that I wanted this show to live in the tension of emotions that felt very contemporary. I wanted to eliminate any distance between then and now. And I just knew the best way to serve this story was exactly how Lin wanted to serve this story, which is that this show should have a cast which reflects the world we live in.”

Hamilton addresses revolution, political partisanship and mudslinging, immigration and questions of belonging – issues still making headlines.

Kail admits he did not appreciate how relevant the production’s themes would feel in 2017. “We had no idea,” he said. “All you can do is try to react to the moments that exist in front of you. But back in 2009, when Lin wrote the first song, there was no Brexit, there was no Trump and there was no knowledge of what would happen.”

One of the show’s most popular lines with US and UK audiences is “immigrants, we get the job done”, which is greeted with a raucous cry of support every night. Kail refuses to be drawn into whether events such as Donald Trump’s racially divisive presidency and the UK’s withdrawal from the EU make Hamilton feel ever more vital and political.

“All I know is that the show feels different from one day to the next,” he said. “The news cycle is such that it is so fast-moving that lines that never really resonated or had a weight to them, all of a sudden emerge.

“But that’s a testament to the specificity of Lin’s writing, because the words have always been the same for the last few years, but we are different and the world is different, so people just interact with it in a different way. There’s no one thing we want people to take from it; there’s no one message of the show.”

Lin-Manuel Miranda (centre) performs with the cast of Hamilton in New York. Photograph: Joan Marcus/AP

Though it has been around for only two years, Hamilton has established itself as one of the biggest stage shows of the century. The announcement that it would make its first international transfer to London was met with a buzz that exceeded any previous production – tickets sold out instantly and resell for more than £3,000.

Hamilton originated as a single song inspired by Miranda’s reading of a biography on the founding father. Composed in 2009, My Shot was first performed during a visit to the White House. It wasn’t until Kail watched the song being performed to an audience at a New York benefit gig in a small theatre in 2012, that he experienced his “thunderclap moment”.

“I saw the audience just quivering. You could feel the hairs on the backs of everyone’s necks was standing up – it really felt like something amazing was happening,” Kail said. “It was a thunderclap moment because I realised there was a version of this that could be done live. I didn’t know what it looked like, I didn’t know what it was going to be, but I knew Lin had to keep writing.”

Even then he did not realise the pair had discovered a groundbreaking formula, spawning a show that would go on sweep the Tony awards and win a Pulitzer prize. “All I knew was that it was the best thing that Lin had written, and the best thing that Lin has written is something to be reckoned with,” Kail said.

During the show’s transfer from Broadway to the West End, Kail and Miranda did not take the easy route. Instead of casting experienced actors in the meatiest roles, they spent almost a year seeking newcomers. The pair announced – to the surprise of many – that a recent drama school graduate would star as Hamilton.

Yet, according to Kail, it was not an impressive CV they were looking for but bravery. And when Westman walked in “we could just sense it”.

“He just filled the room and nothing frightened him,” he added. “And to play Hamilton, what you need above all is fearlessness.”

• This article was amended on 21 December 2017. An earlier version said that one of the most popular lines in the show was “immigrants, they get the job done”. This has been corrected to say “immigrants, we get the job done”.

