Lizzie and Sally may be the core duo, but Kirkwood offers her secondary characters the best material. Susannah Perkins is wonderful as Mary, one of several comic-relief characters, notable for her dim-witted sweetness (she “does not know which glove belongs on which hand,” Lizzie says); Ann Harada shines as a lusty, menopausal mischief-maker, who takes great joy in humiliating the humorless bailiff. Sarah (Hannah Cabell) is a mute jury member who forces herself to speak after years of silence; her hoarse confession that she has seen a cloven-hoofed woman, spitting on blackberries to make them sour, is the dense, dark heart of “The Welkin.” Sarah has, until her outburst, seemed affable and sane, and when she tells the others not only that she met a devil-woman but that the demon delivered Sarah’s baby, the women all accept her testimony as though it makes perfect sense. It’s like the moment in a village-gone-bad thriller, say, “Midsommar” or “The Wicker Man,” when you realize that everybody’s in on it. A sweet face is no guarantee that the mind behind it isn’t wriggling like a bag of snakes.