Scarlett Johansson as a seductive man-eating alien freaks out Venice film festival

US star Scarlett Johansson spooked the Venice film festival Tuesday with her performance as an alien in human form who hunts men, seduces and feeds on them.

"Under the Skin", by British director Jonathan Glazer, plays with light and polished dark surfaces to evoke a sleek alien environment, which is juxtaposed with shots of Glasgow streets and the Scottish highlands.

Johansson's character Laura observes humans with a clinical interest, devoid of impulses such as self-preservation or the emotional drive to save others -- as shown in one chilling scene where she leaves a toddler to drown as she stalks her male prey.

Slowly, she begins to comprehend human feelings and explore her sexuality beyond the drive to feed, tentatively forming her first relationship -- only to discover that the hunter has become the hunted.

The film eludes the science-fiction label, feeling more like a blend of gritty documentary and thriller, flirting with horror without the gore.

"We wanted to make a film about looking at the world through an alien eye. It's about how Laura feels," said Glazer, who based the screenplay on Michel Faber's 2000 novel of the same name.

Johansson said she found it difficult at first to get into her character.

"It became very clear early on, after about three or four takes, that any ideas I had about how to go about playing the character were completely irrelevant. It took a couple of weeks to find our footing," she said.

"The challenge was playing a thing, an 'it', which had no pre-conceived ideas about humanity. The character starts out as this blank canvas, then little cracks appear. I was slowly discovering my character's identity as we went along," she added.

In order to capture real life as viewed by Laura as faithfully as possible, producer James Wilson said the crew had used hidden cameras "so we could film Scarlett in the world, without the world knowing it was being filmed."

Covert cameras -- some so tiny they had to be sourced specially for the film -- were placed in Laura's van, the streets, a shopping centre and a nightclub.

"The cameras are very much an important ingredient in the film. They allow us to watch Scarlett's character disturb reality. Her character is the only lie in the film," Glazer said, adding they had mainly used non-professional actors.

Johansson described working in that environment as "terrifying".

"I was afraid of how people would react. It was amazing how different each take was," she said.

"When my character falls over in the street, for example, some people rushed to help, others walked straight by or stopped to take photographs."

The toughest part of the shoot was filming the final scene in the wet and windy highlands, she said.

"The forest was hell. It was like Scotland was trying to spew us out. It was harsh, brutal, honestly terrifying."