Zodiac star Jake Gyllenhaal has praised the "genius" of his co-star Robert Downey Junior while promoting the movie at the Cannes film festival.
The murder thriller, directed by David FinCher, is one of 22 titles competing for the festival's coveted Palme d'Or prize this year.
Gyllenhaal stars as San Francisco Chronicle cartoonist Robert Graysmith in the film, which is based on the true story of the newspaper man's obsession with identifying a serial killer who stalked California in the 1960s and was known as the Zodiac.
Following a press screening of the film, Gyllenhaal told reporters that acting alongside Downey was "like playing jazz with somebody".
Downey, who plays newspaper crime writer Paul Avery in Zodiac, had around "500 to 750" ideas a minute, Gyllenhaal explained.
"Some people would call that Madness. I would call that genius," he added.
Meanwhile an unknown Briton has received critical praise after his film debut, which tells the story of Joy Division singer Ian Curtis, was screened at Cannes.
Novice actor Sam Riley revealed that he had been "working in a warehouse in Leeds folding shirts" when he was cast to star in Control, which follows Curtis' rise to fame in the iconic group until his suicide in 1980.
The movie, also a first for director Anton Corbijn and starring Samantha Morton, opened the 'director's fortnight' at Cannes.
The festival, now in its 60th year, runs until May 27th.