"The Making of Star Trek: The Motion Picture" listed the short list of candidates for Decker. One of them was a young Andrew Robinson (the future Garak) .
Good he did not get the role. At the time, Robinson was very well known as the "Killer" from the original Dirty Harry (Warner Brothers, 1971), so i'm not sure his turning up as Decker would have been anything other than a visual distraction for a public that remembered the villain from a big movie then only a few years old.