Mike Leigh's new film about the celebrated British artist JMW Turner, has been lauded by critics at Cannes, with some suggesting it may be in line for prizes at the festival and beyond.
"Come awards time, expect Mr Turner to be harrumphing his way to the stage," Total Film's Jamie Graham said.
The film, Leigh's first feature in four years, received its gala premiere on Thursday.
The director's two-and-a-half hour opus, which dramatises episodes from Turner's life in 19th Century London and Margate, is his first period biopic since 1999's Topsy-Turvy, about Gilbert and Sullivan.
However Leigh said the process of making it was "no different" than making his fictional dramas Naked, Life is Sweet and Secrets and Lies.
"You can read all the books in world and do research for a million years, but it doesn't make things happen in front of the camera," the 71-year-old told reporters.
"You still have to create a characterisation and breathe flesh and blood into it. Turner is obviously a great artist, one of the great painters of all time, anywhere.
"I felt there was scope for a fascinating film because of the tension between this very mortal - and in some ways flawed and inspired - individual and his epic work."
Spall, one of Leigh's regular collaborators, said he was ideally cast as Turner because "he was a funny-looking, fat little man and so am I".
"Genius is not always in the most romantic of packages," the actor continued. "Most geniuses are strange and are often odd-looking sociopaths."
He said Turner was "slightly brutish - a man with love in his heart but not sure where to put it".
In addition to his paintings, the film deals with Turner's personal life and his relationships with women - among them his housekeeper, a Margate widow and two daughters he declines to acknowledge.
The Guardian's Peter Bradshaw said Mr Turner - one of 18 titles in contention for this year's Palme d'Or award - represented "another triumph" for Leigh and Spall.
"Every scene... is expertly managed; every comic line and funny moment adroitly presented and every performance given with intelligence and love," he said in his five-star review.
In his own five-star rave, The Telegraph's Robbie Collin said Spall "gives what's probably the finest performance in his career".
Spall, 57, also drew praise from Screen International's Jonathan Romney, who said he was "magnificent" in a film that "may be the most entertaining art biopic yet made".
The premiere of Leigh's film coincides with the issue of a set of Royal Mail stamps in which Secrets and Lies features alongside nine other British titles.
The director said it was "great" his earlier film had made it onto a stamp - albeit one with a value of £1.28, which meant he was unlikely to use it himself.
The 67th Cannes Film Festival continues until 25 May.