The film, starring Elizabeth Berkley as an exotic dancer in Las Vegas, was universally panned by critics after its release in 1995 and went on to become a cult favourite.
But MacLachlan, who played nightclub boss Zack Carey, admits he went into production believing Showgirls would be a provocative hit - and it wasn't until he viewed the finished edit that he finally realised it wasn't his best work.
He tells Britain's Metro newspaper, "I went into that film with the best of intentions. (Director) Paul Verhoeven and (writer) Joe Eszterhas were at the top of their game and I trusted they'd deliver something that would be controversial and hard-hitting. It turned into something completely different...
"Only when it was assembled and I saw it for the first time, I thought, 'Oh boy, this isn't going to be pretty.' Everyone has their Showgirls. We remember the great films actors have been in and the rest get forgotten. But occasionally, people like to revisit the ones that get swept aside."
But MacLachlan admits he's pleased the movie has an underground fanbase, adding, "The fun of it is it's found its own niche in an unexpected way. People still get a kick out of it. I remember the experience and kind of laugh at the movie. At the time, the finished film came as a surprise... We all went into it with absolute conviction. Had we gone into it thinking we were making a camp classic we'd have ruined the camp element. It had to be taken seriously as we were filming it."