“The Fault in Our Stars” is above all a movie about cancer. Cancer provides the butt of the film’s most caustic jokes, provides the magnetic pull that first draws its star-crossed couple together, and provides the power with which the story eventually starts to squeeze its viewers’ tear ducts like water balloons in a pressure cooker. As such, it walks a knife’s edge between heart-on-sleeve sensitivity and crass exploitation for its entire running time, and the fact that it largely stays on the right side of that divide has to mark it as a success. Soulfully acted, especially by a never-better Shailene Woodley, and several degrees smarter than most films aimed at teenagers, this Fox melodrama ought to strike a resonant chord with young audiences.