Movie mogul Francis Ford Coppola will always be indebted to Martin Sheen's wife for allowing the actor to return to the set of Apocalypse Now after suffering a heart attack.

The director's labour of love was falling apart and he is convinced that if Janet Templeton had chosen not to let Sheen return to complete the now-acclaimed war film, it would have fallen apart.

"Martin Sheen was in every shot and every scene and he had a very serious heart attack, to the point we didn't know if he would survive," Coppola explained during the 40th anniversary celebration of the movie at New York's Tribeca Film Festival.

"His life was more important than the movie. The fact that he survived was wonderful news, but had his wife decided to pull him from the film, they would've closed the film, and the film would never have been finished.

"Fortunately she protected him and his love of acting and just took him away for several months before he came back."

The moviemaker kept shooting around Sheen's absence, using his brother as a stand-in: "He had his physique," Coppola explains.

The film legend admits the big regret he still has to this day about Sheen's health battle was a stupid remark he made in his wife Eleanor's behind-the-scenes Apocalypse Now documentary, Heart of Darkness.

"At one point the rumour was getting out that he had died and that's when I said in Ellie's documentary, 'He's not dead unless I say he's dead!'" the director recalls. "I was so embarrassed that I said that."

The film, which also featured Marlon Brando, went on to become a classic, but Coppola admits even he wasn't sure the project would be any kind of a hit.

"There were so many press stories saying the film would never open and they were calling it Apocalypse When," he explains. "It got accepted at the Cannes Film Festival as a work in progress. It wasn't finished. My theory was that at least we showed it. To my amazement we won the Palme d'Or."