Chaim Potok's novel about two Jewish boys in wartime Brooklyn is radical about the word "friend": "You think a friend is an easy thing to be?" one of the characters asks. This adaptation of the book by Potok himself (who died last year) and playwright Aaron Posner gets a simple, elegant local premiere by A Traveling Jewish Theatre. A secular boy named Reuven Malter befriends Danny Saunders, the sidelocked son of a Hasidic rabbi, after getting hit in the eye by Danny during a neighborhood baseball game. The eminent Rabbi Saunders has to approve of any non-Hasidic pal of his son's, so the new friendship moves through formal stages, and Reuven learns that the Saunders house is ritualistically silent: The rabbi and his son don't talk, for spiritual reasons. Reuven has to play intermediary, and being a friend turns out to be not easy at all. Zac Jaffee is excellent as Reuven -- impetuous, young, sometimes insolent -- and Gabriel Carter plays a stiff, shy, but convincingly brilliant Danny. Anthony Fusco as Reuven's father and David Kudler as the rabbi also turn in fine performances. The play loses its shape a little at the end, but overall Aaron Davidman has directed a tight coming-of-age drama, on a plain stage backed with evocative black-and-white slides of old Brooklyn.