The actor played Bobby Ewing on the CBS series from 1978 to 1991, and will also appear in TNT's new revamp.
Duffy told TV Series Finale: "I actually did not think I would be playing Bobby Ewing again. And over the years, more and more, I thought I'd never do it again. Over the years, scripts were presented and people would say, 'Aw, we should do this again'. A script would be written and we would look at it and it would be atrocious."
Duffy added that bringing back the original Dallas cast for the new series was a "necessity".
"[The producers are] smart enough to realise that what made Dallas successful, among many other things, was the fact that it's a family drama," he explained. "You don't need a ton of outside characters all the time. You don't need... the weekly villain or the weekly good guy, or the love interest. It's all about family."
Duffy also revealed that his character Bobby will serve as "the patriarch... of the Ewing family" in the new Dallas.
"He's matured 20 years since we saw him," said the actor. "And that's the reality. We've all matured 20 years since last you saw us, and we just take up the family drama 20 years later."
Dallas will premiere on TNT in the summer of 2012.
> Larry Hagman: 'Dallas relaunch feels amazing'
Watch the trailer for TNT's Dallas below: