Rhys Ifans and Laura Linney on Parkinson's drama 'Onwards and Sideways'

Rhys Ifans and Laura Linney never did a chemistry read before filming Onwards and Sideways, the BBC Parkinson's drama that reaches UK cinemas on 6 November and BBC One and iPlayer later this year. The omission is striking. On screen, Ifans nearly throttles Linney during night terrors; she collapses in his arms, bankrupted by a gambling addiction. Both crises stem from the condition afflicting their characters.

Directed by John Madden (Shakespeare in Love) and set in Norfolk, the film stars Ifans as Tony, a teacher, and Linney as Emma, a musician. "It's odd," Ifans told reporters in London. "You learn so much about each other so quickly. I couldn't tell you what kind of car Laura drives. What her home looks like. But we both know how we feel about death and dying."

Linney, 62, is a triple-Oscar nominee whose credits run from The Truman Show (1998) through Mystic River and Love Actually (both 2003) to Ozark and The Big C. Ifans, 59, broke out in 1997's Twin Town, played Spike in Notting Hill (1999) and Howard Marks in Mr Nice (2010). Both skipped the audition formality and went straight to dance classes instead, waltzing together in London to research dance therapy for people with Parkinson's.

How did the actors research Parkinson's?

In Brooklyn, Linney met David Leventhal of the Mark Morris Dance Group, who runs the Dance for PD programme. "That's the mothership of this wonderful organisation that now has offshoots all over the world," she said. She also observed weekly classes at Juilliard, her alma mater. "How brave it makes people. How it opens up the imagination, and then the body follows somehow, when it shouldn't," she said of the sessions, describing the effect of the music stopping as "shattering in a good and a sad way".

Ifans attended weekly sessions at Pontio, an arts centre in Bangor, north-west Wales, where a harpist accompanied the dancing and a sheepdog support dog ran loose. "It was peak Welshness!" he said, adding: "You'd be sitting next to a nuclear scientist and a school cleaner. Both dancing to the same tune and wrestling with the same diagnosis."

The writer is Paul Mayhew-Archer, 73, co-creator of The Vicar of Dibley, diagnosed with Parkinson's at 58 after noticing his handwriting had become small and unclear. He is a founder of the Movers and Shakers podcast alongside fellow Parkinson's sufferers including Jeremy Paxman and Mark Mardell. "As soon as I said that, I knew I wanted to be funny about Parkinson's, because I saw no point in being serious," he said of the joke from his own diagnosis that appears in Rory Kinnear's doctor scene.

What does the film change for representation?

Parkinson's affects 166,000 people in the UK, with one diagnosis every 20 minutes, according to Parkinson's UK. The condition has more than 40 symptoms, including shaking, stiffness and freezing in doorways, and its medication can trigger compulsive spending and gambling. Physiotherapy specialists the Neuro Heroes coached the actors. "Parkinson's was like a physical dialect for us," Ifans said. "We had to learn its physical language."

Mayhew-Archer said people with Parkinson's could not be cast in the lead roles, partly because Linney's character undergoes a medical procedure that stops certain symptoms. "Once you start freezing, you can't stop!" he said with a laugh.

"We had the added responsibility in getting Parkinson's right," Ifans said. "Not just right, but bang-on." Linney described the stakes as "doing it justice"; Ifans called it "huge responsibility" that "brought us closer together". Linney, who joins DC's Lanterns and is filming Comfort and Joy with Colin Firth and Julianne Moore, said of the experience: "This movie rearranged my cellular structure! Most movies don't do that."

The film arrives in cinemas as a rare mainstream portrait of the condition — one its makers hope the Parkinson's community will recognise as bang-on.