The newcomer will star in the leading part in Sky1's upcoming new retelling of the classic Sinbad stories, which begins on Sunday, July 8.
Knight explained that he was cast with the part straight from drama school in Manchester, but had recently been told to stick to Hollyoaks-style roles.
"We had a week of TV training with this lovely actress. At the end of the week she asked what roles we'd like to do, so I said I'd love to do something really dramatic," he told The Sun.
"She told me, 'You're not bad-looking so you might be good in Hollyoaks or something but anything else, no. It's just not going to happen'.
"She said it in front of the whole class. I was gutted. It's devastating to hear someone say you're not going to be able to do this.
"It just goes to show, never let anyone tell you what you're capable of and not capable of, because they don't know at all."
After he was spotted at drama school, he was invited to a series of London auditions, which included physical training to prove he would be able to handle the role.
"This black belt tae kwon do instructor came in and then started this intense physical workout on camera in front of everyone, which I wasn't expecting.
"I had to do press-ups, sit-ups, then get chased around Leicester Square. People must have thought I had just mugged someone!"
He added: "A week later, I was hanging out with my sister at the arcades. We were playing those games where you hit the moles with a hammer and she was annoying me because she kept distracting me.
"Then my agent phoned to say I had the part. I just remember screaming and then me and my sister jumped around for about half an hour.
"I went from going along to an audition with no expectations, to getting the part in the space of two weeks. I didn't even get to go to my graduation ceremony because we had already started filming."
The series also stars Lost's Naveen Andrews, Sophie Okonedo, Timothy Spall and Orla Brady.
Watch a trailer for Sinbad below: