The 34-year-old actor was to start filming 'Fifty Shades...' five days after finishing 'Sons of Anarchy', where he played psychopath.
Hunnam said he needed a break between the two projects, reported Us magazine.
"I was going to finish playing a psychopath who'd just lost his wife (in 'Sons...'), and five days later I'd be on set playing Christian Grey. I was like, 'This is going to be a ... Disaster.' It was the opposite of how I've tried to ground my career, not stretch myself too thin, and always do my homework," he said.
Ultimately, the role of Christian Grey went to 'Once Upon a Time' actor Jamie Dornan, who will star opposite Dakota Johnson in the erotic-romance, set to hit theatres on February 14, 2015.