Doing It for the Art: Hitchcock went to a lot of trouble to try and avoid the film's twists being revealed. Most notably, he went about buying up copies of the source novel out of his own pocket.
Enforced Method Acting: Hitchcock allowed both Perkins and Leigh to improvise their roles as long as it didn't involve moving the camera. Norman's habit of munching candy is an example. Though it's only an urban legend that he arranged for the shower to suddenly go cold to get the appropriate screams out of Leigh.
Genre-Killer: Many film historians consider Psycho to the be movie that killed Film Noir, as the purpose of the first hour or so is to continuously set up and subvert the tropes of that genre.
Playing with Character Type: Bates initially appears to be the same sort of character Perkins was known for playing up to that time - a likeable, socially awkward supporting role. This makes the Twist Ending all the more shocking.
Real-Life Relative: The brief appearance of young Norman in Psycho II was by Anthony Perkins' son, Oz.
Look closely after the psychiatrist scene, when the cop takes the blanket to Norman. See the other, non-speaking cop who's standing guard outside the door? It's a young Ted Baxter!
Star-Derailing Role: Subverted yet played straight: History will tell you that Psycho was not a box-office nor a critical failure, and it's obviously considered one of the best films ever made. However, many consider it to be the film that simultaneously heightened and ruined Anthony Perkins's career as an actor because he was subject to typecasting afterwards, and most moviegoers only knew him as Norman Bates. Perkins never had any backlash because of it, but plenty of his fans will vouch Perkins was one of the most underrated actors in Hollywood history, and Psycho is to blame.
Ironically enough, at the time Psycho was Perkins Playing Against Type. After Psycho, if there was a movie where he wasn't playing a "psycho," it was him playing against type again.
The same applies to Janet Leigh. Although not subject to typecasting afterwards, pretty much all she is really remembered for was the shower scene.
Throw It In: Location shooting (for the scene where Marion drives out of town with the money) was done in downtown Phoenix in December 1959. After Christmas decorations were discovered to be visible in the footage, a graphic was added to the beginning of the film setting the date as "Friday, December the Eleventh".
Also, Norman's Character Tic of eating candy was a suggestion of Perkins's that Hitchcock happened to like.