In my movie collection database, I personally classify movies like;

Se7en as a Thriller.
Army of Darkness as a Horror/Comedy.
Last House on the Left as a Thriller/Horror.
Blade 1-3, as Comicbook adaptation/Action/Horror.
Shaun of the Dead as a Horror/Comedy.
Constantine as Horror/Comicbook adaptation/Action.

...and so on.

The movies with multiple genre listings are in order of "relativity" for search and cross referance purposes. I will use UP to four genre catagories.