Writer-director Jonathan Levine.

“Warm Bodies” is all about tone, that most difficult of movie components to get right. Levine, who proved some skill with tonal mash-ups in “The Wackness” (2008) and “50/50” (2011), says that’s what Summit Entertainment — the studio also responsible for the “Twilight” series — was looking for when it came to him with Isaac Marion’s 2011 young-adult novel, a post-apocalyptic spin on “Romeo and Juliet.”