In the book Slimed: An Oral History of Nickelodeon's Golden Age, Mitchell Kriegman explained the concept of Sam climbing in Clarissa's window removed the need for him to interact with Clarissa's parents downstairs and "was a way to get him in her bedroom and start interacting faster. And it was also a way to show that they had this real friendship that wasn't about anything sexual. They were friends, and I wanted to keep it pre-sexual."
During the run of the show, several changes are added to the house/set. Clarissa's bedroom window gets a window seat, a back staircase is added in the kitchen as well as a window seat in the kitchen, a guest room and Marshall's study are added, and the front staircase railing was painted white.
According to the book Slimed: An Oral History of Nickelodeon's Golden Age, Mitchell Kriegman had a rule that the color purple was not allowed anywhere in the show: Clarissa's clothes, furniture, scenery, etc.; though by his own admission it was often sneaked into the show.
As Melissa Joan Hart never had time to attend school functions like a normal teenager, the cast and crew of the show held a personal graduation ceremony for her. She was given a diploma for 'Nickelodeon High School', named valedictorian of one and voted "most likely to have her own series".
The theme song is a variation of DJ Mark The 45 King's production of Queen Latifah's "Dance for Me."
Melissa Joan Hart auditioned for Blossom around the same time as this show. She apparently waited for the call as to whether she would play Clarissa or Six. Luckily for her, Clarissa was the one she preferred.
In the pilot episode, Marshall says after Sam goes out the kitchen window "Why doesn't that boy ever use the door?" Sam during most of the show's run, used Clarissa's bedroom window to enter the house.
Clarissa has posters of They Might Be Giants on her wall because they were Melissa Joan Hart's favourite band at the time.
The slang term 'OBKB' came about because it was used by Melissa Joan Hart and her friends.
There was supposed to be a spin-off named "Clarissa Now", which featured her going to college.
The crew from the show Roundhouse stole the famous Big Orange Couch from the Nickelodeon Studio Tour in Orlando and hid it in the prop room for Clarissa Explains It All. The missing couch was quite the controversy and the authorities were even called to investigate its theft.
The famous ladder that Sam uses to climb into Clarissa's window was actually only about 3 rungs high. Even though Clarissa's bedroom is on the second floor, the set was not elevated at all. Sean O'Neal would have to lay on the floor outside the window, get on his knees and pretend to climb up the ladder.
Emily Hart claims that her mother Paula Hart, who was one of the producers on the show, constantly fought the executives' attempts to make Clarissa look more glamorous - wanting to have her eyebrows plucked and more make-up applied.