Actually, I strongly disagree that David never changes throughout the movie.
In the beginning scene, Professor Hobby (played by William Hurt), announces to his production team that he feels that they have plateaued in their progress on the development of artificial beings. He states that the mechas still have one flaw that still separates them from behaving like a human: they have no emotions, no feelings, no sense of self. He proves this point by stabbing a mecha in the hand and asking how it felt. “How did you feel when I did that? Were you shocked? What did it do to your feelings?” The mecha replied, “You did it to my hand.”
Hobby believes that this flaw is an opportunity for creating the next step in the evolutionary development of the mechas. He wants to build a mecha that can feel, that can experience emotions, and he believes that the foundation of these emotions is love. He states that by developing a mecha that can genuinely love and experience love, that this emotional interaction will be the way that a mecha can do things no mecha has ever had before: develop a subconscious, from which a personality can gradually emerge. He says that this could even lead to a mecha being able to dream.
David is the first prototype mecha built with this goal in mind. In the first act of the movie, David is definitely not quite human-like. He sits ramrod straight in a chair. His smile has a strange, forced quality to it. He rarely takes the initiative, always reacting to what others are doing.
As the movie progresses, David becomes more and more human-like in his emotional affect. The way he talks gradually becomes more natural. His outburst during the killing of his doppleganger is the first time we see real anger. This scene is perhaps the most important one in the second act. Earlier in the movie, David is told during the Flesh Fair sequence that he is unique, one of a kind. David incorporates that into his vision of himself, and sees his sense of uniqueness as a justification of why Monica would love him. The sight of another David that looks like him and sounds like him threatened that sense of uniqueness. Unlike an ordinary mecha, however, David has developed emotions, and now he is experiencing a new emotion for the first time: anger. He has developed the emotion of jealousy, and uses it to destroy what he perceives as a rival for Monica’s affection. Because this killing was based on his own sense of self, it can be said that David’s actions were based on having developed a strong sense of self. This scene is the first solid evidence that David has become more than just a mere mecha.
The look of complete despair on his face when he sees the room full of Davids indicates that he is experiencing that emotion for the first time. During his scene with Dr. Hobby, he is slouched in the chair, a pose that he never adopts earlier in the film. (And of course there is a line during this scene that perfectly foreshadows David’s discovery in the next room: David asks, “Am I one of a kind?” Hobby replies, “My son was one of a kind. You are the first of your kind.”)
During the final scenes, which is the day David spends with the revived Monica, the quality of David’s interaction is markedly different from the beginning of the movie. His smile is natural. The way he talks to Monica is more natural. The way he laughs is more natural. He takes the initiative and prepares coffee for Monica. For all intents and purposes, he finally becomes indistinguishable from a real human boy.
I’ve already mentioned how the character of David changes with regards to dealing with the mortality of Monica. The third act deals with the earlier issue of David’s anxiety that Monica has a limited life span. By the end of the movie, David has shown enough development that he is able to handle the concept of Monica’s mortality.
The final scene shows Monica passing away in her bed after falling asleep, and then David falling asleep for the first time in his life. We are informed by the narrator that David does something else for the first time in his life: he dreams, and thus fulfills the final part of Hobby’s ambition…to build a robot that dreams.
He changes plenty, and that’s consistent with the movie’s overarching story of the evolution of robots.