The Hangover Part III is out later in the month and the Hollywood Reporter has an oral history of the making of the first two movies.
HELMS: I was always the nervous Nelly about those jokes. Zach was going to get arrested for the baby thing.
PHILLIPS: Jerking the baby off at Caesars.
GALIFIANAKIS: I did it first with the doll that was just sitting there while we were setting up the shot. I showed Todd, and he goes, “Let’s go ask the parents if we can do that.” (Laughter.) I’m like, “No.”
PHILLIPS: I waited for the [baby’s] mom to go upstairs because the mom was a little bit more not into stuff like that. I go to the dad: “It would be funny if Zach pretends to do this. Would you have a problem with that?” And he literally goes: “[My wife is] going to be gone for a half-hour. Can you do it in the next half-hour?”
COOPER: “Can you jerk my kid off in a half-hour?” (Laughter.)