Venice, Italy - While researching an idea for a movie, German director Werner Herzog visited a US trailer-park to interview a man who spent eight years in a mental asylum for killing his mother with an antique sabre.

"I could tell he was still insane and dangerous," the legendary director said.

Then something on the wall of the man's decrepit trailer sent a chill down his spine: a poster of his own film, Aguirre, the Wrath of God, displayed together with a candle and a crucifix.

The scene persuaded Herzog - who in 2006 was shot by an unknown sniper whilst giving a TV interview - to cut short his visit and to never return.

Herzog shared this anecdote at the Venice Film Festival where he was talking about My Son, My Son What Have Ye Done? which premiered in the Italian lagoon city.

The film is inspired by the real-life story of Mark Yavorsky a San Diego resident who was reportedly a talented acting student before he committed a murder that eerily resembled a scene out of the Greek tragedy "Orestes,".

In the play by Euripedes, a son slays a mother in revenge for the death of his father. At college Yavorsky had been cast for the lead role but was apparently dropped from the cast because of his erratic behaviour.

Herzog's sole meeting with Yavorsky took place a few years before the man's death in 2003.

My Son, My Son, What Have Ye Done? was selected in competition in Venice together with Herzog's Bad Lieutenant: Port of Call New Orleans - a rare occasion in which the world's oldest film festival has chosen to run two films by the same director in the same year.

Also drawing the two films together is the destructive obsessions the main characters in both films display and which to some degree they share with others who populate many of Herzog's most famous films.

These include the crazed Aguirre, who leads his band of Spanish conquistadores to doom in the South American jungle.

Michael Shannon plays lead in My Son, My Son, What Have Ye Done? while in Bad Lieutenant, Nicholas Cage fills the role of the police officer whose life is derailing because of his addiction to drugs and gambling.

Herzog praised both actors and said they shared an intensity displayed by his infamous leading man and compatriot, the late Klaus Kinski.

The turbulent relationship between the 67-year-old Herzog, a veteran of more than 40 films whose approach to moviemaking has been described as compulsive, and the volatile and sometimes violent Kinski, has become part of filmmaking lore.

The frequent fall-outs between the two on the set of Aguirre notoriously degenerated into threats involving guns.

But in Venice, Herzog dismissed the notion that actors have to be a bit crazy themselves to play extreme characters.

He then jokingly cast doubt on his ability to be the arbiter of what mental stability is all about, declaring: "I can say with certainty that I'm the only sane person at this film festival." (dpa)