Michael J. Fox returns to the small screen this spring starring opposite good friend Denis Leary in four episodes of "Rescue Me" -- but why is he in a wheelchair?

"Me playing a paralyzed guy is, like, hilarious because I'm the opposite of paralyzed," jokes the former "Family Ties" and "Spin City" star, who has been battling Parkinson's disease for 10 years now in public, "so it's tricky just to be still."

The upbeat Fox plays a self-described "dark little nasty guy who is embittered by his disability" in the season five episodes, but adds, "I could easily be this [character], were things to happen differently in my life."

Instead, the Hollywood star has been detailing his courageous battle with Parkinson's in a book due this spring called Always Looking Up: The Adventures of An Incurable Optimist. He tells our own Thea Andrews, "If you're never down, you'll never know how good it feels to be up. … I have been spending the last year and a half or so writing a book about optimism and hope and that certain level of acceptance that helps you get through life and accept every day on its own terms."

Fox adds that he doesn't actively look for work anymore, but his life is kept busy by the Michael J. Fox Foundation for Parkinson's Research, inspiring others and fighting the disease that struck him nearly 20 years ago, when he was diagnosed in 1991.

Watch ET for more with Michael J. Fox!