While Beyonce’s performance as Etta James in “Cadillac Records” won critical acclaim, the movie itself hit a sour note at the box office.  So for her next role, the actress/singer turned to a genre that’s traditionally held more mass appeal: the crazed stalker thriller.  In “Obsessed” (which opens nationwide on Friday), she plays the wife of Idris Elba, who’s the object of temp worker Ali Larter’s unwanted overtures.  Beyonce explained to us why she wanted to take this role. (CLICK ON THE MEDIA BAR TO HEAR BEYONCE)