During casting, when asked to take a seat, Robin Williams sat in the chair upside down. Producer Garry Marshall selected him because "He was the only alien to audition."
Many of the gags seen on the show were on-the-spot improvisations by Robin Williams, and later by Williams and Jonathan Winters. The improvisations proved to be so effective and popular that the series' writers soon included specific sections in the scripts where Williams was allowed to perform freely, marked as "Robin goes off here." If you pay attention to Pam Dawber, you can often see her having difficulty not laughing at the ad-libs.
Jay Leno auditioned for the role of Remo DaVinci. According to Leno, he was told by an ABC executive that his face would "frighten little children".
The show was an immediate ratings hit, finishing number three overall during its first season. For the second season, ABC moved the show's time slot to Sunday in an attempt to counter-program CBS' Sunday comedy programming, notably Archie Bunker's Place (1979). Almost immediately, ratings took a sharp decline, and even after returning to the show's original Thursday night time slot midway through season two, the ratings never fully recovered.
Mork's "Nanu Nanu" hand gesture greetings was inspired by Mr. Spock's "Live long and prosper" hand salute from Star Trek, a show which Robin Williams loved. Plus, Mork's Orkan space suit had earlier been a costume from The Savage Curtain (1969).