There are two foci of action in Conor McPherson’s The Seafarer, currently showing in an often brilliant production at American Stage. The first and less interesting concerns the protagonist Sharky Harkin, his blind brother Richard, and their friends Ivan Curry and Nicky Giblin. When The Seafarer is about any two or more of these personages, the dialogue is wonderfully realistic, the relationships are credible and comic, but there’s little development or change to hold our interest. The second focus – the one you don’t dare ignore – is on Sharky and “Mr. Lockhart,” the elegant, quiet-spoken poker player who just may be the Devil. Unfortunately, this storyline doesn’t get half the stage time of the other one, so we find ourselves repeatedly waiting for the supernatural to return and redeem the natural. When it does, The Seafarer is riveting, breathtakingly suspenseful. When it doesn’t, we still admire the play – the superb acting alone is enough to win our approval – but we can’t help but feel impatient. When will another Sharky/Lockhart scene begin?