Knepper, who previously starred in Prison Break and Chase, has joined the show as a character who begins chasing Frank (William H Macy) because of a scam they were both involved in.
Speaking to Give Me My Remote, Knepper joked that his Shameless character is "cookie cutter" compared to his previous roles.
"Simple, compared to these other guys," he said. "But the great thing about that show and why I think it is going to be a big hit is that all these characters have to be believable. They have to be real people with real lives."
He continued: "They're doing dastardly things to each other but they're funny. I'm still going through hell with, 'Is this working?' as we all do and then in the end you go, 'It's a comedy'. It's going to make people laugh instead of run away from me."
The US version of Shameless will premiere on January 9 at 10pm on Showtime.