Any longtime Hulk fan would have realized that something nefarious was about to go down when Bruce Banner (Edward Norton) was communicating with a Dr. Samuel Sterns about a cure for the Hulk. In the comics, Sterns is the green-skinned villain with the elongated cranium named The Leader. But in the movie, he’s still a simple doctor looking to solve life’s mysteries, while also storing samples of the Hulk’s blood for a rainy day.
When he's attacked in his laboratory and infected with Banner’s irradiated blood, his head begins to grow for just a second and a sick grin takes over his face. It was brief and went without explanation, but comic book fans knew exactly what the filmmakers were hinting at.