Melanie Lynskey and Kate Winslet were so strongly into their roles that they would interact with each other as their characters off-screen.
A picture on the wall in Pauline's bedroom is a photograph of the real Juliet Hulme.
All of the journal voice-overs are direct from real diary entries made by Pauline Parker. The characters in the stories (if not the stories themselves) and make-believe world are also authentic.
Melanie Lynskey was cast as Pauline Parker two weeks before filming began. Co-writer Dame Fran Walsh discovered her at the last minute while scouring local high schools for Parker lookalikes. Upon being cast, Lynskey was shown Kate Winslet's audition video and was told: "This is how good you have you be."