Michael Jackson always said that he wished he could live on stage, and in Michael Jackson's This Is It, there isn't a moment when he looks less than comfortably and pleasurably at home there. On the vast, half-empty, often darkened proscenium of the Staples Center in Los Angeles, where we see him in bare-bones videotaped rehearsals for the 50 London concerts that he never lived to perform, Jackson moves lightly and easily, with his herky-jerky demon-marionette grace. On the rare occasions when he's not focused on dance moves and has nothing to do but sing, as in a soaring interlude of ''Human Nature'' or a version of ''I Want You Back'' that he tosses off with affection for his child-superstar pluck, the music pours out of him like sunlight.