What drove Rogen crazy was Efron's body.
"The first time I saw him without his shirt on, my first instinct was, there was something wrong with him," the Knocked Up star told Conan O'Brien.
"We had to keep writing into the scenes that I touched his body," he added. "I would look at it and want to just reach out and touch it."
Promos for the actors' upcoming film emphasize their physical disparity – both men pose shirtless, Rogen's paunch contrasting with Efron's gym-given physique.
Neighbors hits theaters May 9.