MacLachlan's unfortunately stiff performance can be viewed in all its glory on Blu-ray, out today for the first time, though you're likely to be distracted by the magnificent sets and costumes, which are the real attractions. To be fair, however, MacLachlan was asked to play a reserved, enigmatic character, a scion of a royal family. And, at least as scripted by Lynch, Paul Atreides never came into focus, which may help explain why MacLachlan looked so lost in the role. Another factor is that we've never seen Lynch's original cut, which was bowdlerized before its theatrical release. Perhaps everything would have made sense, and MacLachlan might have emerged in a fully-formed performance, if Lynch had his way. Glenn Erickson (AKA DVD Savant) has a detailed explanation of the circumstances involved, as well as a review of the new Blu-ray.
Lynch used MacLachlan to much better effect in their next joint outing, 1986's Blue Velvet. Other actors debuting in starring roles fared much better, some even earning Academy Award nominations. But MacLachlan's debut remains a personal favorite, the young actor retaining his earnest dignity in a movie that feels today as though it were made in a dead language.