Although it was no big secret -- the two are both listed as BB-8's vocal consultants in the film's credits -- neither has spoken out about their involvement until now.
Hader said it took a couple of tries to perfect BB-8's machine-sounding voice.
"At first I tried doing a voice, but we all agreed it sounded too human," he explained.
Schwartz, who had been keeping his involvement in "Star Wars: The Force Awakens" close to his chest, tweeted that "nothing makes me happier" than being BB-8's voice consultant. He added, "This movie is amazing."
"Star Wars: The Force Awakens" is produced by Lucasfilm, a subsidiary of Disney, which is the parent company of ABC News.