Shepard’s great strengths are his Kerouac-like robustness, even silliness, and his poetic resistance to explaining his artistic impulses, as he digs and digs to get at something that means something to his consciousness—especially his unresolved relationship with his father. But the only character in “Heartless” who seems to have that kind of pull on him is Mable Murphy (brilliantly portrayed by Lois Smith)