"I was fed up with the usual detective routine," Bård Breien told the BBC's Jon Sopel. "I said if I'm ever going to make a film about a detective, he's going to have Down's Syndrome."
Hofsø, 33, had previously on starred in an amateur film made by his brother and sister, spent a year preparing for the part, putting in such a good performance that it challenges viewers' assumptions about what Down's Syndrome people are capable of.
"A lot of people told me that it might end up on the bad taste side of it," Breien said. "It was a struggle to make him act so well that he became a beautiful main film character."
In September, Hofsø won an award for best actor at the Fantastic Fest film festival in Texas
Hofsø said that if the film is a success, he was considering a film career.
"We have no plans to make number two yet, but it is a good idea. If this film is going to be success, maybe," he said.
The film premieres in Norway next month.