Have everyone select a book for silent reading time.  Set a timer or announce when twenty minutes have elapsed.  At that point, require that each student verbally give a thirty-second book report.  Instruct them to offer plot summary information, interesting characterization, and any themes that have been introduced in their current reading selections.  It might also be useful to require that each student, in order, is responsible to ask one question of a reader before delivering his or her verbal book report.