Hollywood is easing up on the pyrotechnics after a May for the ages, offering a pair of openers with R ratings, modest theater counts and expectations to match. Universal’s “Knocked Up” is the most prominent entry. Reteaming director Judd Apatow with “40-Year-Old Virgin” co-star Seth Rogen, the R-rated comedy follows two twentysomethings (Rogen, Katherine Heigl) whose drunken hook-up results in a pregnancy. Film will bow in 2,873 locations, 34% fewer than the number at which “Pirates of the Caribbean: At World’s End” opened last weekend. After racking up more than $400 million worldwide in its long-weekend launch, “At World’s End” should easily retain the weekend crown. The other R-rated hopeful is MGM’s “Mr. Brooks,” opening at 2,453. Kevin Costner is stretching to play his first completely evil character, a serial killer.