The sound matches the mood
Facial expression(s)
In between scenes the image on the screen doesn't fade to black it just cuts to a black screen.
The costumes ; both the main characters wear jeans that look faded/worn out, dean wears either a normal T-shirt or a polo shirt with a leather jacket or a normal top over it. He has a small amount of stubble this makes him look rugged this is important because dean is a strong, protective, loyal, determined, car crazy and can be slightly emotionally detached so his costume reflects those parts of his character. He also wears a leather jacket that is too big for him this is important because it was his dads jacket and he is trying to fill his dads shoes

Sam on the other hand usually wears checkered button up shirts with a normal jacket over the top of it and worn out jeans. The checkered shirt reflects his brainy nature.
The font used on the end credits looks like typewriter font and is white so it stands out on the black background, this pairs well with the creepy background sound of the credits
Season one episode 21 "Devils Trap"
In one of the scenes where they are in the car talking to each other the camera cuts between Sam and Dean using side shots of their faces which stand out against that black background.
Lights flash against their faces which creates more shadows and matches the dark atmosphere.
From the end of one scene and in to another their is the sound of a guitar and drums in the background until Dean and Bobby start talking, shadows play on bobby's face.
The sound of a dog frantically barking outside interrupts their conversation then suddenly stops this puts everyone on edge a few seconds later someone that has been possessed by a demon kicks the door open and storms in.
During the interrogation scene there is a fire burning in the background, it could be interpreted as representing hell.
In this particular scene the use of shadows, sparse lighting, eerie background music, facial expressions and fire add to the atmosphere of the scene.
Contacts are used to give people 'demon eyes' (usually black) that show when the demon possessing someone reveals itself to one of the hunters when its trapped.
The camera focuses on dean and Sam who is in the background goes blurry and vice versa when the camera switches to Sam.
Near the end when Deans talking about killing possessed people and what he would do to protect Sam and their dad you can hear the sound of a piano in the background.
At one point the lights flicker and the wind picks up and because the characters are already on edge this makes the atmosphere even tenser.
Also when Dean realizes that his father John has been possessed he backs away from him and points a gun at him at this point johns eyes flash amber this not only shows that he's possessed but also shows that its by a very powerful demon.
Season two episode one "In my time"
In this episode dean is fighting for his life and has an out of body experience this allows the camera to cut between spirit dean and dean lying on his hospital