Speaking to news.com.au, the 43 year-old revealed that he turned down the part in the award-winning film as he was already tied to another movie at the time.
"I had a contract with 'The Black Dahlia' that I had to film, so I had to drop out of it."
Hartnett added: "It was a different film altogether, it was me and Joaquin Phoenix. But they went on to do it with Heath (Ledger) and Jake (Gyllenhaal)."
The actor then described their version as "a different film altogether."
Set in Wyoming in the 60's, the story follows two cowboys who fall in love over the course of a movie but have to keep their sexuality secret from the outside world.
The film, directed by Ang Lee, was released to critical acclaim, winning three Golden Globes and four Academy awards.
"I've always wanted to kiss Joaquin, so that's my biggest regret," Hartnett joked.
Hartnett also spoke of his decision to leave Hollywood, turning down some major roles in the process in order to get away from the fame. He was offered the roles of both Superman and Christopher Nolan's Batman.
Speaking on why he left the spotlight he said, "There were a few (catalysts) – journalists were less kind to celebrities back then, there were no outlets like Twitter or Instagram to voice your own version of things. You were at the mercy of journalists really, unless you played that game very cleverly," Hartnett said.
"And I was pretty young to be playing cleverly, so would find myself at the darker end of that spectrum where you'd have people talking about what you do on a moment-to-moment basis in a non-flattering way, and I just wasn't interested in having that be my life."
Hartnett has returned in the years since and is once again working on big productions.
