Michael J. Fox and Eric Stoltz have developed a "friendly correspondence" 40 years after Back To The Future.

The 64-year-old star replaced Eric, also 64, as Marty McFly in the iconic 1985 sci-fi blockbuster six weeks into production, and the pair hadn't spoken until Michael decided to send him a letter decades after the change.

As reported by Entertainment Weekly, he wrote in new memoir Future Boy: "Eric has maintained his silence on the subject for 40 years, so I was prepared for the likelihood that he'd prefer to keep it that way."

In the letter, he quipped: "If your answer is 'p*** off and leave me alone'… That works, too."

Instead, Eric sent back a "beautifully written reply", jokingly telling Michael to "p*** off and leave me alone".

Michael continued: "Eric was thoughtful about my outreach, and although he respectfully declined to participate in the book, he seemed open to the idea of getting together."

Since then, the pair have developed a friendship of sorts, and one that Michael never saw coming.

He wrote: "In the months since meeting, Eric and I have maintained a friendly correspondence - volleys back and forth between like-minded actors and dads, offering up recent movies we've loved, the latest adventures with our kids and an occasional detour into politics. "His emails are reliably witty and always fun to read [and] a reminder that some of the best parts of our future can come from the past."

The pair "immediately fell into an easy dialogue" when they first met, discussing everything including their careers, family life and respective "trips through the space-time continuum".

Michael recalled: "[Stoltz entered] with a smile, and we quickly acknowledged that neither of us had an issue with the other.

"What transpired on Back to the Future had not made us enemies or fated rivals; we were just two dedicated actors who had poured equal amounts of energy into the same role.

"The rest had nothing to do with us. As it turned out, we had much more in common than our spin as Marty."