Inadvertently done, possibly. Disappointed because he couldn't get Hayley Mills for the role of Nancy, the director gave Lori Martin a hard time during filming, which probably resulted in her nailing every scene as the terrified, vulnerable girl.
Polly Bergen suffered minor bruises in a scene where her character struggles with Cady. Robert Mitchum was supposed to drag her through various doors on the set, but a crewmember mistakenly left all those doors locked, so that when Mitchum forced Bergen through the doors, she was actually being used as a ram to push them open.
While filming their fight scene, Gregory Peck accidentally hit Robert Mitchum for real. Mitchum, knowing that Peck didn't mean to and ever the professional, refused to break character and continued filming the scene. However, upon entering his trailer, Mitchum said that he "literally collapsed" due to the impact of the punch and said that he felt it for days afterwards.
Recycled Set: The interior of the hotel Cady takes Diane to is the same set that was used in Psycho two years earlier.
Throw It In!: The scene with the egg was added by the director on the day of filming. Polly Bergen's reactions to Robert Mitchum rubbing the eggs on her was real. And the parts where he drags her through the house? That happened because several doors wouldn't open and he had to force his way through in order to keep the scene going. (Too bad it resulted in her incurring some genuine injuries as well.)
Robert De Niro helped costume designer Rita Ryack choose Cady's clothing.
Originally in the scene where Cady puts handcuffs on Lori, she was supposed to start freaking out, Illeana Douglas was the one who came up with the idea of having her character laughing and trying to play along instead.
In the original script, Leigh only met Max Cady at the end. Jessica Lange suggested the scene where they talk outside her house be added to the script, because she felt there should be a meeting between the two before the climax.
Robert De Niro and Juliette Lewis also both came up with the idea of her sucking his thumb during the auditorium scene. What you see in the final movie is the first take.
Robert De Niro worked extensively in preparation for this film so that he would be as physically imposing as possible. Nick Nolte, on the other hand, lost weight to make his character seem more vulnerable (especially because Nolte is actually several inches taller than De Niro).
De Niro also went several steps further by getting tattooed for real (albeit with vegetable dye so that they'd eventually fade away) and paid a dentist $20,000 to mess up his teeth to play long-term prison inmate Cady. Afterwards, it cost him $25,000 to have the damage fixed.