Heartthrob Michael Fassbender admitted Thursday that love "made me get my shit together" as he debuted his latest film with co-star and girlfriend Alicia Vikander -- a weepie which has Venice reaching for the kleenex.

Heartfelt performances from the real-life couple in "The Light Between Oceans", a suspenseful melodrama, had critics at the Venice film festival in tears, despite some grumbles that the plot was far-fetched and the emotion piled on too thick.

Fassbender, 39, and Vikander, 27, fell in love on set after spending a month together on location on a remote island in New Zealand where they suffered lightning storms and ferocious winds to get into character.

The Irish-German heartthrob, famed for his roles in "Shame" and "12 Years a Slave", quipped that as soon as he met the young Swedish actress, he knew he had to get his act together. 

"I was scared when Alicia came, she was so fierce and hungry. I remembered how I had been when I was starting out. I really felt like I had to up my game, be as present as she was," he told journalists.

Vikander, who won an Academy Award for Best Supporting Actress for her performance in "The Danish Girl", said she was "very nervous" when she found out she was to play across from "the most brilliant actor out there".

The couple, who waved to fans as they swept up to the Lido island off Venice in a water taxi, were expected to whip up a storm when they hit the evening's red carpet at the glamorous beach-side festival.

- 'Between truth and love' -

The latest film from Derek Cianfrance is largely set on Janus Rock, a storm-lashed island 100 miles (160 kilometres) off the coast of Australia, in the immediate aftermath of World War I.

Fassbender plays Western Front survivor Tom Sherbourne who signs up to become the guardian of Janus lighthouse in a bid to buy the time and silence needed to recover from the horrors of war.

On his way there, he meets Isabel (Vikander) who although in mourning for her dead brothers, is full of the life Sherbourne seems to have lost, and the pair fall in love and marry, moving to the only house on Janus together.

Emmy Award-winning director of photography Adam Arkapaw -- the man who shot Fassbender in "Macbeth" and the upcoming "Assassin's Creed" -- captures the film's moods in desert-like seas and bleeding sunsets.

At first it seems an idyllic spot to start a family, but tragedy hits. Desperate for a child, they find themselves forced to choose between emotions and morals when they discover a baby washed up on the island.

Tom realises during a trip back to the mainland that the infant's mother is alive and mourning her offspring, and the film draws heavily on the parable of Solomon, where two women both claim to be an infant's real parent.

"This movie is a battle between truth and love," said Cianfrance, best known for his features "Blue Valentine" and "The Place Beyond the Pines".

He said he had made it his mission to "tell family stories, the secrets in our homes" and the flick, based on Australian novelist M.L. Stedman's work of the same name, "seemed like a movie I was born to make".