The former Knots Landing star, 66, reprised his role as Gary Ewing when the character returned to Texas for the funeral of his sibling, oil tycoon J.R. Ewing, in the revamped series.
Shackelford's return to the show's iconic Southfork Ranch after 22 years should have marked a defining moment in his acting career, but he was so sick with a virus he picked up on set, he can barely even remember the scenes he shot.
He tells UltimateDallas.com, "You'd think I'd be able to remember but I have to tell you I was so f**king sick when I did this thing I could barely stand up. My memory is a little faulty. I had some kind of sinus thing that I still have, I might add... Everybody's got the same thing going around and it lingers forever."
Shackelford also faced a double disappointment as he was unable to watch his scenes when his comeback episodes aired in the U.S. because he cringes when he sees himself on screen.
The actor adds, "No, no (I don't watch myself on Tv). God no. I was barely on when I first started in television and I used to watch it and I reached a point where it was just counterproductive. I would watch it and I'd say, 'It's just dumb. My God, why did I do that?' or 'That's not what I meant'.
"It was too frustrating. I don't really do it for that. I do it for the immediacy of it, the feeling of it. That's where I take my sustenance is from the actual doing of it. I don't care about afterward... I don't know, it makes my life easier if I don't have to watch it. I'm also slightly embarrassed by it... I do (get critical), yeah. Then I start thinking 'Jesus, why did I ever work again after that scene? Why would anybody hire me?'. It's not good for me."
Shackelford was one of the original Dallas actors who returned to the revamped series this year (13) to mourn J.R. Ewing's passing after the actor who played him, Larry Hagman, died in 2012.