"I knew absolutely nothing about Bobby Darin before this film, but now I'm a huge fan," said Bosworth, wearing a black cocktail dress by Yves Saint-Laurent for the occasion. "There is darkness and tragedy in the story, and it was a dream for me to land this part."
The film was a labor of love for Spacey, 45. Besides being on-screen, the two-time Oscar winner produced, directed and co-wrote the film. He also sang all 18 songs on the soundtrack.
Spacey has long been fascinated by Darin, who rose from poverty to become one of America's biggest stars of the 1950s and '60s with hits such as "Mack The Knife," "Dream Lover," "Splish Splash" and "Beyond The Sea."
Darin married Dee, and they became Hollywood's golden couple. But Darin died in 1973, at age 37, from a heart condition brought on by rheumatic fever he had as a child.
"Making this film was the most fun I've ever had in my entire life," Spacey said. "Bobby Darin was one of the greatest entertainers the world has ever known, and yet because he died young he's been kind of forgotten. This is my tribute to someone I think was a remarkable talent."
Spacey said Dee, who now lives as a virtual recluse in Los Angeles, approved of the film. "She called me last week and said she loved it," he said.