Robert Pattinson was a stealth guest at a premiere of his on-again girlfriend’s new film “On the Road” Saturday night. Spies say Pattinson “snuck into the theater” at the AFI Fest screening at Grauman’s Chinese theater to support Kristen Stewart, but he managed to evade detection. “Nobody knew he was there . . . he did not want to distract from her night,” our spy said. After the film, the pair arrived together at an after-party at the Hollywood Roosevelt Hotel and hung in the event’s Audi SkyLounge. “They held hands and stayed very late,” a source said. Earlier in the day, the pair promoted their upcoming “Twilight” film in Los Angeles.