AUSTIN, Texas — They came expecting a civilized, one-on-one discussion, but they got what some attendees described as “a train wreck.”

Ballroom A of the Austin Convention Center was packed to capacity Sunday evening for an hour-long interview with Facebook CEO Mark Zuckerberg, the keynote speaker at this year’s South by Southwest Interactive festival. The 23-year-old billionaire founder of the social networking site was interviewed on stage by author and journalist Sarah Lacy. Using her unique, friendly style of interviewing — closer to two friends chatting than a straight question-and-answer session — Lacy tried to get the notoriously tight-lipped Zuckerberg to open up. But the discussion rarely strayed beyond the usual business fare and eventually descended into a string of awkward moments punctuated by the audience’s heckling.