Mat Whitecross screened a copy of the film, in which Lord of the Rings star Andy Serkis portrays the late Hit Me With Your Rhythm Stick singer, for Hollywood executives at the Sundance film festival in Utah in January (10) - and he was stunned when they presumed Dury was a fictional character.
Now premiering the much-anticipated biopic at the Tribeca Film Festival in New York, Whitecross tells WENN, "I met with some people who'd seen the film, and I didn't really understand the gist of the conversation. Their questions didn't make any sense.
"They kept asking me, 'Who wrote the music?' After a while I realised, 'They don't know it's a real person'."
But Whitecross is still confident the subject matter will appeal to movie fans, even if they've never heard of Dury, adding, "It didn't matter to them (the movie bosses) that they'd never heard of Ian. They found the story engaging."