''I run a roller coaster emotionally,'' he said, ''from wishing the film would come out and all this would explode so I'd know where I stand, to wanting to crawl back into anonymity.''

His girlfriend, he said, worked with Christopher Reeve a few months ago and asked the actor to help her understand what she could expect when ''Dune'' was released. ''She wanted to know what was happening to me,'' Mr. MacLachlan said. ''Reeve told her I would be impossible to live with for the next year.'' He paused. ''She and I have already broken up.''

Son of a stockbroker, born and raised in Yakima, Wash., and a graduate of the University of Washington's theater-arts program, Mr. MacLachlan was performing in ''Tartuffe'' at a Seattle theater when he was asked to audition for ''Dune.'' Out of school less than a year, he had already spent seven months at the Oregon Shakespeare Festival with roles that included Romeo in ''Romeo and Juliet.'' Rejected Because of Haircut

Raffaella De Laurentiis, the producer of ''Dune,'' and David Lynch, the movie's director, had spent almost a year searching for their Paul Atreides. ''We wanted somebody unknown, somebody who had the look of a Messiah, and somebody who could act,'' Miss De Laurentiis said. ''In Kyle's first screentest, we could see that the kid could act. You have to act or nobody will believe you're the new Messiah. The look was there too, but he had a punk haircut that didn't help.''

At first Dino De Laurentiis rejected Mr. MacLachlan because of the haircut, but with a new haircut and another screen test, the role was his.