Sia performs on the main stage on the final night of the Coachella."Is that a llama?" asked a woman next to me before the stage lights went up.Nope -- it was just Sia, wearing her signature two-tone wig and a huge, fluffy dress under which the singer's alter ego of a dancer, Maddie Ziegler, was hiding.Or at least the dancer looked like Ziegler. It was almost impossible to tell.At a Coachella defined by surprise pop-star cameos, Sia's performance Sunday night cleverly flipped the script. As usual, the singer delivered songs such as "Chandelier" and "Elastic Heart" while standing off to the side of the stage, her face hidden beneath that wig.Beside her, dancers including Ziegler (or her lookalike) acted out a series of vignettes that were also playing out on the main stage's enormous video screens.But instead of live close-ups from the stage, the video sequences were actually prerecorded studio versions of the same dances featuring well known actors such as Paul Dano, Tig Notaro and Kristen Wiig.Which meant that the show's guest stars were the folks onscreen, not in the flesh.Or were they? Perhaps that was Wiig onstage after all, dancing her heart out right alongside Ziegler.