scored a blink-and-you’ll-miss-it role in the rom-com as a hard-partier who strips off his clothes, offers drugs to other party guests and checks out a jacuzzi.And here is how the “Saturday Night Live” alum, 28, snagged the quirky walk-on — or jump-on — part and was caught off-guard in the process.“I didn’t even tell him what he’s doing,” film director Jason Orley told Variety on the red carpet at the picture’s Los Angeles premiere on Thursday. “I said, ‘Can you come down for a night?’ and he said ‘Sure!’ And he showed up and I said, ‘Okay, get up on this ledge and take off your clothes.’ And he was like, ‘What are you talking about?’”For the cameo, Davidson was fastened onto the ledge of a roof — wearing only his underwear — and leaped onto protective pads on the ground.