"Real heroes don't wear Spandex," Mel Gibson told Venice Sunday as he unveiled his new World War II drama "Hacksaw Ridge", starring Andrew Garfield of "Spiderman" fame.

The film tells the true story of Desmond Doss (played by Garfield), who enlists and is determined to save lives on the front line as a medic, but refuses to carry a gun on moral grounds.

The flick's title comes from a battleground in Japan at the top of a towering cliff. US soldiers who climb its sheer face are met with bunkers and corpses, as well as Japanese bullets and flame-throwers.

Doss, despite being a conscientious objector, was awarded the Medal of Honor by President Harry S. Truman for single-handedly saving the lives of over 75 of his comrades during the brutal Battle of Okinawa.

While the first hour of the movie is essentially a love story between Doss and his future wife Dorothy (Teresa Palmer), the rest sees the bible-clutching medic first reviled then gradually accepted by the rest of his squadron.

"The man who refuses to touch a weapon and wants to do something much higher than something as venal as killing in a war is a very high calling," Gibson said, adding that he did believe "just wars" exist.

He said that with the violent but essentially uplifting film he hoped to "pay homage to and honour the warrier".

"It's a sad fact that veterans of wars harm themselves afterwards. In Vietnam so many people were killed in the conflict but afterwards over three times as many took their own lives."

His Academy Award-winning "Braveheart" (1995) was famed for its bloody battle scenes, but here Gibson has taken the blowing off of legs and slicing through of guts to an operatic level.

"The important thing with battle and depicting it on screen is to give the impression of chaos and confusion but to be absolutely clear what you want the audience to see," he said.

- 'Like a good Dad' -
"It's all about screen direction and knowing where the players are, you almost have to approach it as a sporting event. If you then put characters into that situation that you have actually come to care for it takes it up a level."

A bearded Gibson, whose first directoral offering in 10 years is in competition for the Golden Lion, said he had felt like "a traffic cop" director, while Garfield said he was more like a mother.

"Mel's really in the scene with you, he can't help it. He's always there with every single actor in every single moment. He's like a good dad or a good mum, with that kind of wonderful nurturing instinct," he joked.

Garfield, who shot to fame with the movie "The Amazing Spider-Man" (2012) before co-producing and starring in the 2014 thriller "99 Homes", said Doss was much more inspiring than the web-weaving hero, whose Spandex costumes prompted Gibson's jibe.

"The fact that this man, who is built as skinnily as I am, dragged men across the most rugged terrain under gun fire, sniper fire, the possibility of motors and shells, and then lowered them down a 75-foot escarpment... that's like when you hear about mothers who lift trucks off babies," said Garfield.

"He had a knowing in his heart and core that he wasn't supposed to take a man's life, but wanted to serve something greater than himself, and found his personal genius path to do that," he said.

He added: "It's a pretty wild time we're in societally speaking. There's a lot of violent uprising and separation and warring ideologies that are plaguing our beautiful planet right now.

"I think Desmond Doss is a symbol who embodies the idea of live and let live no matter what your value system is. You can't really argue with that, I think we could all learn a thing or two from Desmond."