After seeing Eighth Grade, Bo Burnham's enormously affecting new movie, you might assume that a lot of the dialogue was improvised. Most of it was, in fact, carefully scripted, which makes it all the more remarkable: It's been a while since I've heard a screenplay so fully master the awkward, hesitant rhythms of everyday teen speak. Burnham's young characters talk in long, rambling but more-or-less coherent sentences, each thought punctuated with a perfectly timed "um" or "like" or "you know."