Friday afternoon began with possible tragedy: a hostage crisis at Hillary Rodham Clinton’s campaign office in Rochester, N.H. As the incident unfolded, Clinton’s campaign closed its doors and canceled her public appearances. But nightfall brought a happy ending: the campaign workers safe, the man in police custody — and the New York senator flying to the scene to express thanks. The hostage-taking itself offered a rare, if small, genuine drama in a campaign season governed by strict schedules and scripted stump speeches. And as soon as it ended, Clinton took full advantage of the opportunity she had unexpectedly been handed....