The stars of the CBS comedy “The Big Bang Theory,” Jim Parsons and Johnny Galecki, play physicist nerds but “said they did not understand any of the scientific dialogue in the show. … Mr. Parsons memorizes his lines by writing them out longhand and says he is astonished when people ask if the actors on the show ever improvise. ‘To veer away from the scripted dialog is a one-way trip to sudden death,’ he said.”