In his latest book, Michael J. Fox, 64, recounts the miraculous, exhausting circumstances that saw him star in “Back to the Future.”

Eric Stoltz was initially hired to play the hapless Marty McFly, who is accidentally transported back to the 1950s, where he “meets” his future parents.

Fox had been the first choice for the film’s producers — including Steven Spielberg. However, at the time, he was starring on the smash hit sitcom “Family Ties.” The show’s head honcho, Gary David Goldberg, nixed the idea of him taking the movie role.

“[He] said it was impossible to consider me for the part, citing my commitment to ‘Family Ties,’” Fox writes in his new memoir “Future Boy: Back to the Future and My Journey Through the Space-Time Continuum” (Flatiron Books, out Tuesday).

But a few weeks later, Spielberg, a personal friend of Goldberg’s, called him asking for help.

“They had shot for over a month,” Fox writes. “Eric was an immensely talented actor, but the creative team felt that he just wasn’t the right fit for Marty McFly. Spielberg had come back to Gary and begged him to share me.”

Goldberg agreed as long as Fox didn’t let the schedule affect his work playing Alex P. Keaton, an ambitious young Republican with former hippie parents.

The young star ended up working double shifts — shooting “Family Ties” during the day at the Paramount lot, then heading to Universal to film “Back to the Future.” He got just a few hours of sleep each night.

“It would never work today, since our business is layered with lawyers, business affairs, and insurance companies,” his agent, Bob Gersh, later said. “Now a movie project demands two weeks of buffer time on either side of a job.”

Fox also faced a movie cast and crew that weren’t exactly welcoming when he began filming.

Lea Thompson, who played his mother in “Back to the Future,” was “cranky” when they met on set.

Thompson had worked with Stoltz before and was “bitter about how he had been let go … I was the recipient of those feelings,” Fox recalls.

She also later sheepishly admitted to a “certain superiority” over Fox because he was a TV actor while she was a film actor.

But after filming their first scene together — when Marty wakes up in his mother’s bedroom — those feelings quickly vanished as they both realized they were “developing a rapport.”

Fox’s wonderment over how lucky he was didn’t leave him.

During a midnight shoot on “Back to the Future,” the catering team laid out a table brimming with food.

“In my daze, I uttered, ‘Wow. All of this stuff is free,’” Fox writes. “Did I just say that out loud? Already sleep-deprived, my inner broke actor had emerged — the guy who not too long before this moment was picking day-old cookies out of dumpsters behind the bakery.”

The star, who was diagnosed with Parkinson’s disease in 1991, gently writes about Stoltz. “From what I gather, Eric’s Marty was more somber than mine, approached through the lens of Marty’s altered reality at the film’s end,” he notes.

While writing “Future Boy,” Fox reached out to Stoltz, now 64, to see if he would be interested in talking about his “Back to the Future” experience. The “Mask” actor declined but did meet Fox for coffee at his apartment,

“Our two-hour conversation revealed a mutual love for our craft, rooted in classic films,” Fox writes. “In the months since meeting, Eric and I have maintained a friendly correspondence … his emails are reliably witty and always fun to read.”

Fox also reflects on his time on “Family Ties,” which made him a household name.

He had been struggling to get work after moving from Canada when he booked the NBC sitcom, which was originally supposed to focus on his parents, played by Michael Gross and Meredith Baxter.

For the first year, Baxter picked up Fox every day to drive to set together. The young actor didn’t have a car and had been taking the bus to get to work.

With “Back to the Future,” it was apparent early on that it would be a huge hit. The movie tested through the roof.

It opened on the July 4th weekend, less than ten weeks after it wrapped.

Amazingly, Fox missed the movie’s premiere as he was filming “Family Ties” in London. Upon arriving back in Los Angeles, he slipped into a movie theater and watched the film.

It would go on to become the highest-grossing film of 1985 and spawned two spin-off movies and even a Broadway musical.