Less than a year after “I Swear” helped explain the complexities of Tourette’s Syndrome on the big screen, “Onwards and Sideways” is looking to do the same for Parkinson’s Disease.

From director John Madden (“Shakespeare in Love”) and written by Paul Mayhew-Archer (who was diagnosed with Parkinson’s in 2011), the film — released in U.K. cinemas on Friday — stars Welsh screen legend Rhys Ifans as Tony, a much-loved and deeply caring primary school teacher who is calmly told by a doctor he has “five good years” left before Parkinson’s takes over his life. At almost the same time and at the same clinic, musician Emma (Laura Linney) is given the same diagnoses, and the story follows the journey of the pair as they try to manage and come to terms with a disease that, as they’re both painfully aware, is only going to get worse.

Like “I Swear” before it, “Onwards and Sideways” is a film that’s deeply moving, at times upsetting to watch but frequently funny, and also one that brings to light various aspects of an incurable disorder that most viewers probably knew very little about. As it shows, among the various side-effects of the drugs used to help manage Parkinson’s is compulsive spending and gambling.

For Ifans, known for playing Hugh Grant’s raucous housemate in “Notting Hill,” but also for “Mr Nice,” “Not Only But Always” (which saw him win a BAFTA TV award for playing Peter Cook) and numerous other screen and stage roles over a career spanning 30 years, the film — and working so closely with the “heaven-sent” Linney — was unlike anything he’d experienced before.

But, as he explains to Variety, it was also one that came with a deep responsibility to accurately portray a disease that people around the world live with each day. From “consuming everything” he could about the neurological disorder to attending a dance class with sufferers, Ifans details how he learned to “wear the Parkinson’s costume.”

When a script like that lands on your desk, one written so exquisitely about such a sensitive subject, what’s your immediate response?

As an actor, it’s the perfect list: tender, dark, funny, moving, complicated and nuanced. That’s life. I always respond well to anything where laughter and tears are often dancing together.

Preparing for a role like this must have required more care and responsibility than others.

We were representing a vast worldwide demographic of people who live with Parkinson’s day by day, and will live with it for the rest of their lives. It’s something that won’t get better — it evolves. So there was no question that we had to do everything in our power to make the portrayal as authentic, real and tangible as possible. Laura and I both went to great lengths, consuming everything we could about Parkinson’s.

There’s a wonderful podcast called “Movers and Shakers,” which our writer, Paul Mayhew-Archer, is part of. I devoured their entire back catalogue, driving the length and breadth of England and Wales while listening to the experiences of people living with Parkinson’s. I learned about everything, from how to get dressed to coping with sleep, and all the little details in between.

On the physical side, our director John Madden brought in two wonderful women, Laura and Anna, from a group called Neuro Heroes. They’re neurophysiotherapists who work closely with the Parkinson’s community and people with other neurological conditions, helping them understand their bodies and the importance of exercise. They worked with us for two or three weeks before filming began. We mapped it all out and it was daunting and frightening. Both Laura and I wanted to run away. But slowly, it starts to fit. You begin to wear the Parkinson’s costume, if you like, and it starts to feel as though it belongs to you, as much as it can for someone who isn’t living with it every day.

Of course, I’ll never get close to understanding what it truly feels like. The words I’ve heard from people who have Parkinson’s are that you’re constantly under attack from your own body, under siege in many ways. But we had to do our best. So, to give you the short answer: I really did my homework.

I think I read that you also joined a dance class?

Yes. Laura went to some wonderful classes in Juilliard, where people with Parkinson’s move to music. It gives them some respite from their symptoms. Something happens neurologically when music reaches the brain; it almost hacks into your system and gives you a sense of levity.

I went to a wonderful class in North Wales. These classes take place all over the world, with people with Parkinson’s gathering in church halls, ballet studios and other spaces. The one in Wales was particularly close to my heart because, of course, I’m Welsh. But it was an amazing collection of people, both those running the class and those attending. There was a harpist and even a support dog.

It’s incredible to watch a room full of people with Parkinson’s enter, seemingly shackled by their condition, and then see what music and community can do for their minds and bodies. It’s incredibly powerful.

This film is about much more than the disease and what it does to you — it’s also about that community.

Absolutely. I wouldn’t call it a love story. I’d call it a story about love. And that love is spread far and wide.

That wonderful moment at the end with the children in the school saying goodbye to Tony is so moving. I don’t think it’s possible not to cry during that scene.

For me, that’s the moment when I’m fully kebabed by the film. From then on, it’s very difficult to stay in control. It’s a wonderful moment.

My father was a primary school teacher, and I remember his final day at school. I really got a sense of how it must have felt for him. I really wanted to play the best teacher I could. So, for me, the film is twofold: it’s an homage to the courage of people with Parkinson’s, but equally an homage to great teachers.

Parkinson’s is a disease that I don’t think many people really understand. Watching the film, I realized just how little I knew. I had ignorantly assumed it was simply a matter of having the shakes, but I had no idea about the range of neurological symptoms and side-effects, including things like gambling addiction.

There are up to 40 Parkinson’s symptoms. And counting, probably. Everyone gets their own version of the disease. Of course, there’s the symptom we’re all familiar with: shaking. But my character, Tony, doesn’t shake. He experiences a phenomenon where he freezes and is unable to move forward. Then there are the medications that ease these symptoms, but they often come with very strange and disturbing side effects. These can range from gambling and sex addiction to constipation.

That’s a whole other universe that people with Parkinson’s have to navigate, juggle and come to terms with. I think the film really explores that part of a Parkinson’s sufferer’s daily life, including the whole ritual of taking medication at the right time. You’re constantly trying to stay on top of symptoms that could literally floor you.

We’ve had a few beautiful, tender, real and also funny films over the past year about conditions and illnesses that people didn’t always necessarily understand. “I Swear” last year was phenomenal, and now there’s “Elsinore” and this. It feels like there’s a real moment in cinema of films looking to help explain diseases we thought we knew about, but actually don’t.

I think film is a great way of introducing people to these subjects. There’s a line in the film: “You need to know the person behind the symptoms.” Film does that better than almost any other medium because it potentially reaches so many people.

Everyone I’ve spoken to who has seen the film has walked out of the cinema knowing or understanding more about Parkinson’s. For me, that’s job done.

Of course, your scenes with the wonderful Laura (Linney) are fantastic, but I also loved seeing Monica (Dolan). She’s on a real roll at the moment and appearing in so many brilliant films, including “Elsinore.”

Laura is literally heaven-sent. I wouldn’t have been able to achieve what we’ve achieved here without Laura and me being able to make ourselves completely available to each other in a way I’ve never experienced before. We had to be able to say to each other, very early on, how frightened we were about entering into this. Laura has been wonderful.

And then Monica is like the cavalry. There are actors like that: you call on them, they come in and they deliver every time. She’s a master craftswoman. She has the most exquisite timing, humanity and authenticity. I really felt I was in the company of two very special women in this film, between her and Laura. It was a pleasure coming to work every day.

You and Laura hadn’t previously starred alongside each other, but you’ve both appeared in beloved Richard Curtis romantic comedies. Did you discuss your experiences on “Notting Hill” and “Love Actually” at all?

Funnily enough, while we were rehearsing, Richard came and had lunch with us one afternoon. He’s very nosy and wanted to know what we were up to. I describe Richard as Christmas in trousers. He’s a wonderful being. Laura and I would often regale ourselves with fondness and love for Richard. He’s never too far away, and, of course, he was a great friend and collaborator of Paul Mayhew-Archer, who wrote the script. So, yes, Richard was spoken of often and fondly.

See the “Onwards and Sideways” trailer below: