Which genre-defining characteristic do you think is most important when determining a film's genre? Why?

I think there is no one characteristic. There is the way the film looks, the music played with the film, the characters in it, the way they act, the way they interact. There is so much that goes into a genre of film but when certain characteristics, come together is when you can truly determine the genre. Red undertones, sadistic music, evil characters, scary movie. Warm undertones, upbeat music, happy characters, happy movie, perhaps romance or comedy.