Hill stars as a college dropout who agrees to look after three mischievous children as a favor to his mother. He's forced to take kids with him while he scores some drugs for a girl, but when the deal goes bad, the night turns into a wild chase through New York City, where Hill proves his mettle as a babysitter.
Rockwell will play a villainous drug dealer along with J.B. Smoove of "Curb Your Enthusiasm" fame.
Max Records, Kevin Hernandez and newcomer Landry Bender play Hill's troublemaking charges, while Rockwell's "Conviction" co-star Ari Graynor will play the female lead.
David Gordon Green directs the raucous comedy, which boasts a well-received script from Brian Gatewood and Alessandro Tanaka that landed on the 2009 Black List.
Michael De Luca ("The Social Network") is producing the picture, which is currently filming in New York.
In a smart bit of counterprogramming, Fox is scheduled to release the R-rated comedy on July 15, 2011 against the final installment of WB's family-friendly "Harry Potter" franchise. That means "The Sitter" will open just three months after Universal releases Green's latest comedy, "Your Highness."
Rockwell previously worked with Green on the drama "Snow Angels." He next stars opposite Hilary Swank in Tony Goldwyn's legal drama "Conviction," which Fox Searchlight releases this weekend in select cities. He has been earning strong notices for his performance as a Massachusetts man who was wrongfully convicted of murder.
Rockwell is represented by Gersh.