Intertextuality in films is where one film has a particularly famous and popular scene in it that writers take, modify and add into their films. For example, the famous shower scene from 'Psycho' has inspired many other films including 'What Lies Beneath', 'The Stepfather', 'Fatal Attraction' and even a student made film 'Succubus'.
All of these films copy one or more of the main elements of the scene in 'Psycho': a murderer, the knife, the setting of a bathroom. Each film adapts the scene to fit the plot of the film, therefore making necessary changes.
'What Lies Beneath' copies the Mise en Scene of 'Psycho's' shower scene. The scene is still set in the bathroom with the element of the shower being turned on. The camera focuses on the water, similar to the shots from 'Psycho'. Again, there are only two characters, the victim and the attacker. Like in 'Psycho' the victim is female and the attacker is male. The difference between 'What Lies Beneath' and 'Psycho' is that in 'What Lies Beneath' there is no element of the knife and the victim has been placed in the bath by the attacker to drown to death.
'Succubus' is a student made film that also uses the scene from 'Psycho'. Again, this film uses the setting of a bathroom, though this time, its a shower in a locker room. The roles of the victim and attacker are reversed in this film, this time the victim is male and the attacker is female. At first, it seems that the attacker wants to get in the shower with the man but then she produces a knife - another main element from 'Psycho'. This scene copies many of the camera angles that were used in 'Psycho': close-ups of the shower, the knife being used and the victim himself. There is also a close-up of the victims last movement, this was also in 'Psycho'.
'The Stepfather'. Again, this film has the original male attacker and female victim, although this time there are two other people in the house. The victim is chased upstairs into the bathroom by the attacker who is clutching a knife. Here we know that the knife and bathroom element have been 'borrowed' from 'Psycho'. As the man kicks the door, a mirror breaks and shatters on the floor. The woman then picks up a shard of mirror and holds it as a weapon to defend herself. As the man raises the knife to stab her, she stabs him in the neck with the shard of mirror. This is a twist that wasn't used in 'Psycho'. He ends up falling backwards into the bath, pulling down the shower curtain in the process. This is another element from 'Psycho'. Originally, in 'Psycho' the victim breaks the curtain, but this time its the attacker who pulls it down.
All these films have taken the shower scene from 'Psycho' and made it their own, only keeping the one or two of the main elements in their 'shower scene'.