Casting can be a tricky process — it's hard to pick the perfect person to bring a character to life!
Sometimes an actor's not fit for a role — but they're so good that the casting directors NEED to put them in the show, so they actually write the actor their own part.
Like Chris Colfer, who auditioned for Artie on Glee and wasn't quite right for the role — so the producers wrote him an entirely new part.
Or Aubrey Plaza, who the Parks and Recreation casting director thought was the weirdest person she'd ever met. She referred Aubrey to Mike Schur (the show's creator), who then started writing the part of April.
Lexi's part on Euphoria was actually written specifically for Maude Apatow — not based on her audition for the show, but on her performance in Assassination Nation.
One of my favorite examples is Lost — many of the characters were actually written for actors who had made an impression in auditions, and other characters were changed to better fit the actor they wanted to cast.
