If all went as planned, former “Xena: Warrior Princess” star Lucy Lawless was on the set of Fox’s “The X-Files” on Tuesday, to kick off a two-episode stint that could develop into a regular role. “Xena” shut down production in May, after the TV star, 34, had announced that she had planned to take off a year in order to spend time with her daughter, 13, and son, 18 months. Speaking to New York’s Daily News about her appearance on “X-Files,” Lawless said, “I wasn’t looking to be a guest on anybody’s show . . . But then (‘X-Files’ creator) Chris Carter tracked me down when I was vacationing in Hawaii and asked me to do it.” She credited Carter’s “hands-on” approach to the show as her reason for returning to work. She also revealed that the two episodes might actually expand to eight, though her role remains shrouded in secrecy. “You won’t be able to tell if she’s a good or evil force,” the show’s executive producer, Frank Spotnitz, said.