Last week a note on what a pair of good actors can do for a weak romantic script. This week a look at what a crackerjack cast—first-class all the way—can do for a middling contemporary drama. Kramer vs. Kramer, based on a novel by Avery Corman, was written by Robert Benton who also directed. I don’t know the book: the script tells, in lithe dialogue, the story of a New York couple who split. The struggle is not about the divorce, which as such is never glimpsed, it’s about the custody of their small son.