Excerpt from Making Movies:
In the early days of television, when the “kitchen sink” school of realism held sway, we always reached a point where we “explained” the character. Around two-thirds of the way through, someone articulated the psychological truth that made the character the person he was. [Paddy]
Chayefsky and I used to call this the “rubber-ducky” school of drama: “Someone once took his rubber ducky away from him, and that’s why he’s a deranged killer.” That was the fashion then, and with many producers and studios it still is.
I always try to eliminate the rubber-ducky explanations. A character should be clear from his present actions. And his behavior as the picture goes on should reveal the psychological motivations. If the writer has to state the reasons, something’s wrong in the way the character has been written.