In this weekend’s Friends With Benefits, Justin Timberlake takes on his first full leading role. Whether his acting game is as tight as he (and the New York Times) forecast, his likability/trying-very-hard game is, as ever, unparalleled. Last night, Timberlake visited Jimmy Fallon, and the pair performed a second installment of “The History of Rap.†The first time around, in September of 2010, the duo covered everything from “Rapper’s Delight†to Eminem and Soulja Boy. This time around, they included everything from Salt n’ Pepa and Cypress Hill’s “Insane in the Membrane†to “It Takes Two,†“Hey Ya,†and Lil’ Wayne. There’s even a truncated reference to Vanilla Ice, JT’s voice cracking on the high notes of “Hot in Herre,†and, a little past the three-minute mark, Jimmy and JT working out their Dougies. Timberlake’s dance moves skew toward the goofy, but he does have some undeniable swag going on in the pelvis thrusts in “Push It.â€