If you have read The Ruins, Scott Smith’s much-praised 2006 horror novel, you can probably guess that there will come a point at which the movie version succumbs to silliness. And sure enough, that moment arrives about a third of the way in. Without divulging too much about this tale of four hard-partying American tourists whose hike into the Mexican jungle goes disastrously wrong, I’ll only say that there is an unfortunate instance in the film that evokes Little Shop of Horrors. Reading the book, that musical comedy about a man-eating plant never entered my mind.