Acting legend Harrison Ford has praised his Shrinking co-star Michael J. Fox as “the most stubborn optimist I know” as the two shared the stage for a poignant event.

Fox, 65, was diagnosed with young-onset Parkinson’s disease in 1991 – a year after the release of Back To The Future Part III, in which he played the character Marty McFly.

The Canadian-American actor, who has spoken often of the debilitating effects of the condition, was diagnosed at the age of 29 after noticing a trembling in one of his little fingers. He has continued to appear in TV roles in the decades since his diagnosis.

“It’s (acting) really prolonged my life and made my life interesting in a way that I didn’t think it would be to this point. I’m not retired from acting, but if I didn’t act again, it would be for a very good reason. I let Parkinson’s make decisions for me,” Fox told The Hollywood Reporter at an annual philanthropic event this week.

As well as the Back To The Future trilogy, Fox is known for films such as Teen Wolf and Doc Hollywood  and the TV series Spin City.

Most recently, he appeared opposite Ford in the third season of TV comedy Shrinking. Over several episodes, Fox plays Gerry, a Parkinson’s patient who befriends Dr Paul Rhoades, the therapist character portrayed by Ford, who also has the disease.

Fox stars opposite Ford in the latest series of Shrinking. Photo: Apple TV

Ford was at this week’s THR event to present Fox and Deborah W. Brooks, the co-founder of his Parkinson’s foundation, with an award. Since 2000, the group has funded more than $US3 billion ($A4.3 billion) in research into the progressive neurological condition.

“Thirty-five years ago, Michael J. Fox got some very bad news. Most people handed something like that endure it; Michael went right to work,” Ford said.

“Playing Paul Rhoades on Shrinking gave me some understanding of what it means to live with Parkinson’s and it also gave me Michael, as an actor, as a friend and the most stubborn optimist I know. Through him, I’ve seen what his foundation does.”

Shrinking creator Bill Lawrence said his father and grandfather had both been diagnosed with the condition.

“What I was struck by was how it paralysed my family when we heard that stuff, and how quickly Mike made the transition from saying, ‘I’m retiring’ to suddenly working harder than anybody I’ve ever met in my life,” he said.

Fox said that, after his diagnosis, he would anonymously visit chat rooms of Parkinson’s patients and ask what they thought of the celebrity, meaning himself, who had just gone public. Responses ranged from encouraged to ecstatic.

“I thought, ‘well, f— you, I have Parkinson’s and you’re celebrating’,” he said.

“And I thought [it’s] because they’ve just been voiceless for so long. It was considered an old person’s disease — put grandma on the porch and just let her rock until she stops rocking. And, in fact, it didn’t have to be that. A lot of people that are young, they have a lot to offer politically and economically, and as athletes and activists.”

Fox said he had to overcome “vanity” when revealing the impact of the disease on him, adding that he needed to “normalise it”.

He has previously been open about how Parkinson’s has affected his memory and made it difficult to remember dialogue in his acting roles. He has also detailed numerous broken bones from falls over the years.

But he is also very hopeful about the future, sure that “we can do it” and “we can build a drug that will save people’s lives”.

-with AAP