Kline appears to be having the time of his life onstage at the St. James Theatre, where the latest Broadway revival of this 1939 drawing-room comedy opened Wednesday. It's a fast-paced and straightforward production, directed by Moritz von Stuelpnagel, featuring two equally delightful and delighted costars - the goofily irresistible Kristine Nielsen as Essendine's stalwart secretary, Monica, and Kate Burton, steely and wry as his devoted not-quite-ex-wife, Liz - plus a mixed bag of supporting players and a jam-packed Edwardian flat of a set, designed by David Zinn, that gets its own entrance applause.