Michael J. Fox: I Love My Life

Michael J. Fox, who has lived with Parkinson's disease for almost 20 years, has a hard time not moving still because of the degenerative nerve disorder. But now, with a four-part guest spot on Rescue Me, the actor will have face the challenge of staying still to play a paralyzed, wheelchair-bound man.

"The funny part is me playing a paralyzed guy because I am the opposite of paralyzed," he tells Entertainment Tonight in an interview airing this evening. "It's tricky just to even just to be still."

The character is a far cry from the good guys Michael usually plays on screen, which will be another stretch for the veteran actor, one he's looking forward to.

"Dennis Leary called me up and described the character," he says. "He is really dark and misanthropic. He's bitter and nasty. It's really fun."

Although the beloved actor has been struggling with the disease for a long time, Michael has remained optimistic about his condition, saying he's "pretty good all things considered," and penning Always Looking Up: The Adventures of an Incurable Optimist, due out this spring.

"If you're never down you'll never know how good it feels to be up," he tells ET. "It's a great ride and I wouldn't change a thing. I love my life for whatever difficulties there are."

Tune in to ET tonight for more!