One of the original choices for Fezzik was Arnold Schwarzenegger, and one of the first choices for Vizzini was Danny DeVito. But don't feel bad for them. The pair got together a year later to make the movie Twins. Which was almost as good. (Source)

Did you feel like the chemistry between Westley and Buttercup was a little too good to be true? You'd be right. They quite fancied one another in real life, and that definitely translated to the big screen. As to whether they ever rounded any of the bases, their lips are sealed. (Source)

Rob Reiner told Mandy Patinkin that he could have any part in the movie he wanted. That's some offer. We're glad he selected the role he was born to play, and not Buttercup, for example. It would have made a few of those scenes a little weird. (Source)

Wallace Shawn, who played Vizzini, was deathly afraid of heights. He was not uber-thrilled to shoot the scene at the "Cliffs of Insanity," and kinda wigged out a little bit. It was a challenging day for the actor, but at least he brought the insanity. (Source)