Timothy Spall says he often looks at his copy of “Snow Storm” by J.M.W. Turner, which hangs at his home, and thinks, “How on earth did I ever do that?”

Enter Tim Wright, the London-based portrait artist and lecturer who gave Spall a two-year crash course in fine art, and turned the actor into not only a competent painter, but one who could convincingly copy the work of one of the world’s finest masters. “The request was quite astounding really,” laughs Wright.

Spall’s best actor prize at Cannes confirms they got there in the end, but when he first started working with Spall, all bets were off. “You were never going to be able to teach someone to paint exactly like Turner,” says Wright, “because Turner’s a genius, and there was a lifetime of painting involved in making him one.”

So the approach was to school Spall in the basics of fine art and to give him an understanding of how a painter sees the world. “I was lucky that he was so enthusiastic and eager to learn,” he says. “I designed a course for him to follow, starting with life drawing and watercolors, and eventually we moved onto oils and we copied a couple of Turner paintings to see how it was done.”

“Tim taught me the rudiments of all the disciplines,” confirms Spall. “And I got immersed to such a degree. He gave me a Fine Art university course–just me and him, nobody else.”

Mike Leigh is unlikely to have wanted it any other way, but when it comes to Turner’s particular style of painting, there simply wasn’t another approach. “The secret to what Turner did was that he was an incredible observer,” explains Wright. “He looked at the world around him and didn’t take it for granted. Tim got in that mode of questioning what he was looking at as Turner did.”

And then there’s the physicality. “It’s such a physical process, painting,” he continues. “Turner is documented blowing snuff onto the canvas and occasionally using spit. It wouldn’t be unnatural for him to behave like that, and there was a lot of gestural painting to get the right effect of the physicality in his seascapes and skies.”

On screen, watching Spall paint as Turner is electric, especially the scene at the Royal Academy’s Varnishing Day, the annual event at which the masters would display their works in public for the first time and put the finishing touches to them. “Turner would put on a real show,” Wright says. “He would grandstand it, and he would slap down rivals to preserve his brand.”

Watching the film demonstrates, says Wright, the “huge amount and depth of research” that was put into getting the artist right. For Spall, this extended beyond painting lessons. “He was already interested in Turner, but he became obsessed with learning about the man.”

So what we see is the real deal. “You don’t get an impression of Turner, you get an expression. When you see someone painting in a film usually you’re watching a whole portfolio of clichés about artists. We’ve bypassed that. This is a sincere expression of life as a painter.”

Wright and Spall will both exhibit work, alongside Turner originals, at an exhibition at Petworth House in West Sussex, England from January 10 to March 11, 2015. More information can be found at http://www.nationaltrust.org.uk/petworth-house/.