(Adds quotes from actors, director, Kampusch appearance)
By Georgina Prodhan
VIENNA, Feb 25 (Reuters) - A new film based on the story of Austrian kidnap victim Natascha Kampusch shows her being repeatedly raped by the captor who beat and starved her during the eight-and-a-half years that he kept her in a cellar beneath his house.
Kampusch was snatched on her way to school at the age of 10 by Wolfgang Priklopil and held in a windowless cell under his garage near Vienna until she escaped in 2006, causing a sensation in Austria and abroad. Priklopil committed suicide.
Kampusch had always refused to respond to claims that she had had sex with Priklopil, but in a German television interview on her 25th birthday last week said she had decided to reveal the truth because it had leaked out from police files.
The film, "3,096 Days" - based on Kampusch's autobiography of the same name - soberly portrays her captivity in a windowless cellar less than 6 square metres (65 square feet) in area, often deprived of food for days at a time.
The emaciated Kampusch - who weighed just 38 kg (84 pounds) at one point in 2004 - keeps a diary written on toilet paper concealed in a box.
One entry reads: "At least 60 blows in the face. Ten to 15 nausea-inducing fist blows to the head. One strike with the fist with full weight to my right ear."
The movie shows occasional moments that approach tenderness, such as when Priklopil presents her with a cake for her 18th birthday or buys her a dress as a gift - but then immediately goes on to chide her for not knowing how to waltz with him.
Antonia Campbell-Hughes, who plays the teenaged Kampusch, said she had tried to portray "the strength of someone's soul, the ability of people to survive... but also the grey areas within a relationship that people don't necessarily understand."
The British actress said she had not met Kampusch during the making of the film or since. "It was a very isolated time, it was a bubble of time, and I wanted to keep that very focused," she told journalists as she arrived for the Vienna premiere.
Kampusch herself attended the premiere, looking composed as she posed for pictures but declining to give interviews.
In an interview with Germany's Bild Zeitung last week, she said: "Yes, I did recognise myself, although the reality was even worse. But one can't really show that in the cinema, since it wasn't supposed to be a horror film."
The movie, made at the Constantin Film studios in Bavaria, Germany, also stars Amy Pidgeon as the 10-year-old Kampusch and Danish actor Thure Lindhardt as Priklopil.
"I focused mainly on playing the human being because... we have to remember it was a human being. Monsters do not exist, they're only in cartoons," Lindhart said.
"It became clear to me that it's a story about survival, and it's a story about surviving eight years of hell. If that story can be told then I can also play the bad guy."
The director was German-American Sherry Hormann, who made her English-language debut with the 2009 move "Desert Flower", an adaptation of the autobiography of Somali-born model and anti-female circumcision activist Waris Dirie.
"I'm a mother and I wonder at the strength of this child, and it was important for me to tell this story from a different perspective, to tell how this child using her own strength could survive this atrocious martyrdom," Hormann said.
The Kampusch case was followed two years later by that of Josef Fritzl, an Austrian who held his daughter captive in a cellar for 24 years and fathered seven children with her.
The crimes prompted soul-searching about the Austrian psyche, and questions as to how the authorities and neighbours could have let such crimes go undetected for so long.
The film goes on general release on Thursday. (Reporting by Georgina Prodhan, Editing by Paul Casciato)