There's no one perfect answer to this question, as it varies from cover to cover and from film to film.
I try to think about the film itself, its theme, its subject matter, and its visuals. What aspects of the film do I want to represent on my cover?
Just some examples from my gallery...
Furious 7: Metal textures to suggest the cars, grunge stone to suggest the asphalt/streets, and a mix of contrasting cool/warm colors to relate to the film's somber/intense moments, respectively.
Hocus Pocus: Leather textures to suggest the spellbook, paper textures to suggest its pages, grunge textures to suggest the age of the 300-year-old witches. Browns and oranges to suggest the autumn setting. Purple skies to suggest a "magical" night sky. Green accents to indicate the magic/lightning seen in the film. The color scheme uses the secondary colors (orange, green, purple). Other witch-related imagery scattered throughout.
Cabin in the Woods: Grunge paper to suggest the old items in the cellar, particularly the diary. Browns and wooden texture to suggest the cabin itself. Reds and splatters to indicate blood and death. Hints of honeycomb patterns to hint at the "wall illusions" seen in the film.
Smokey and the Bandit: Metals to suggest cars. Grunge paper and maps to suggest the road trip/chase aspect of the film. Asphalt textures to suggest roads. Stars and stripes (and faded reds, whites, blues) to give a southern patriotic (and slightly confederate) vibe. Other car-related imagery (hubcaps, license plates) scattered throughout.
Scream: Grunge textures. Dark red skies to suggest blood/death/danger. Red slash lines to suggest stab wounds and blood trails. Shattered glass to suggest broken windows, chaotic lives. Also, glass being broken by a loud, piercing scream. Subtle TV static and film strips to relate to the movies within the movies aspect of the film.
Sorry for the lengthy post, but I wanted to give different examples of how thinking about the in-depth details of a film can provide a lot of imagery and visuals to work with.
Best of luck!
Really cool to see your insight on how you go about creating the great covers you do Kernie! Really interesting.