LONDON—Mr. Turner is Mike Leigh’s masterpiece. The biopic about one of the great landscape painters, J.M.W. Turner, is told on an epic scale and visually stunning without sacrificing Leigh’s genius for putting the very soul of his characters on screen.

He has been in the director’s chair for more than 40 years, working with the best actors and creating some of the richest characters in British cinema, but convincing enough investors to let him make Mr. Turner has taken half of his working life—more than two decades.

“People said the budget was too big for an arty or art house picture,” he said, speaking from a small leather armchair in a bright Soho office. “That may still be true. I think the jury is out.”

Box office takings remain to be seen but the critical verdict has been overwhelmingly positive with Oscar buzz for Timothy Spall, who plays Turner and joins fellow Brits Eddie Redmayne and Benedict Cumberbatch among the favourites for best actor. The movie cost a little over $13 million to make, not a vast sum for a filmmaker with seven Oscar nominations to his name. The battle to secure that funding is testament to the difficulty in making truly independent movies.

“We say: ‘Give us some money, we don’t know what it is going to be about, we will not discuss casting, and you will see the film when it is finished.’ And only one of two things happens; either they say ‘great, here’s the bread?’ or they say ‘fuck off.’ And when that happens, we don’t make the film—that has happened a lot,” he said, laughing.

Despite his enduring film industry success, Leigh has no time for Hollywood, and he is dismissive of the franchise-milking directors working for the major studios. “They are motivated by insecurity, fear, lack of imagination and above all, a lack of flair,” he said.

For Leigh, it would be impossible to make his intimate, searing, and deeply funny films with the shadow of studio executives looming over his creations. “We don’t have committees of people on the set crowding around the monitor after every take and arguing the toss. They don’t bugger about with the editing. We don’t get people making us re-cut the film and get a more sexy or commercial ending or all that stuff,” he said.

Leigh’s filmmaking style is the stuff of legend; he always reserves six months for rehearsals before shooting begins; there is no script; no predetermined structure. He works with the actors individually to explore their characters and build up their lives without telling them anything about the other characters or the overall plot of the film. Only then are they are introduced to one another for hundreds of improvisation sessions led by Leigh. This is where the scenes that will eventually make up the movie are forged.

Marion Bailey, a Mike Leigh veteran who plays Turner’s partner in the latest film, explained the process. “He works with you to make every sentence, every beat, every word. You go over and over until it’s right, you learn it in your head—nothing is written. Only then does he bring in the crew and you shoot it.”

The relationships between the characters have evolved almost organically, certainly gradually. It’s obvious to Leigh that this is how a film should be made. “We make the characters absolutely solid and so what you see in the film is the tip of the iceberg but you believe in it. The characterization is not going to be merely a surface thing that has been created just for this moment that the camera is turned on,” he said.

It’s only after these long, improvised scene-building sessions that Leigh puts together the narrative structure of the film. “In the end the journey of making the film is the journey of discovery as to what film is. And that is why it is so important that you haven’t got wankers interfering with it with their fingers in everything,” he said.

Mr. Turner is set in the artist’s last 25 years, during his late era work which was more experimental and more controversial to the critics. Spall plays him brilliantly as a grumbling, grunting beast of a man whose sensitivity and kindness emerges slowly. The character emerged slowly, as well. Spall spent two years training to paint in preparation for the role.

“People make a lot about this but the truth of the matter is; if you’re going to bloody well play Turner in a movie, it is pretty obvious that you’re going to get out there and do a bit of painting,” Leigh said. “If you’re going to play a blacksmith you have to learn how to shoe a horse. I didn’t want to have to do that daft old fashion thing of having a close up of somebody else’s hands.”

Spall was rewarded for his hard work with the best actor prize at the Cannes Film Festival.

He wasn’t the only one working hard. Bailey, who is also dating the director, said working on a Leigh production was incredibly arduous. It’s not just a question of learning your lines, he may ask you to come back tomorrow with research on some area of history, or having invented a couple of new family members. “It’s demanding but you know that when you start,” she said. “He’s not like a slave-driver but he demands a lot of himself so you have to step up to the plate.”

His idiosyncratic methods help to extract great performances from his actors, many of whom return over and over again to work with him. Sally Hawkins, the star of Happy Go Lucky, won the Golden Globe for best actress, while the leading women in Secrets & Lies (Brenda Blethyn) and Vera Drake (Imelda Staunton) were both nominated for Oscars.

“They bloody well deserved it and in my view they were better than the people who won. Tim [Spall] could well be nominated and not win because Hollywood films tend to win. Whoever wins will no doubt be good, but I doubt that they will be better than Tim,” Leigh said.

In the film, Turner begins to think about how his work will be seen after his demise. When asked to evaluate his own work, Leigh was a little more reticent. He said it was too soon to judge Mr Turner and certainly too soon to describe late era Leigh—“I have not died yet!”

“On the whole I kind of quite like my films without watching them every night like Gloria Swanson in Sunset Boulevard,” he said. “But I do actually think they are alright. A lot of filmmakers say ‘I can’t watch myself’ and, very often it is because the film that got made isn’t the film they wanted to make because a few people had their hands all over it and fucked it up.”