Donald Margulies, one of the most accomplished American playwrights, is probably less famous than he should be, in spite of a Pulitzer Prize for Dinner With Friends (2000) and notable achievements such as Collected Stories (1996) and Time Stands Still (2010). It’s unlikely that the shoestring production of Sight Unseen at the charming Access Theater will add significantly to his reputation, but an exceptional cast and astute direction make the quaint, four-flight walkup worthwhile for theater lovers. (An elevator is also available.)