c.) Sound

There is both diegetic sound (sound that exists in the world of the film) and non-diegetic sound (sound/music that is added to create mood and feeling appropriate to the intended meaning of the scene) in the opening scene.
Even though it seems like most of this scene is in silence, when you are watching and listening closely you can hear the diegetic sound coming through - like the growl of the cat in the background, the faint sound of leaves hitting the ground and when you hear Eli's very clear and deep breaths.
As an example of non-diegetic sound there is also low, scary music that is added to the scene.

All of this creates the feeling of creepiness and nervousness because we associate these sounds as scary noises and are expecting something loud to come and give us a fright.

The growl of the cat is very creepy and strange as it doesn't sound like a normal cat. It brings attention to the cat being different looking and unusual.
When we hear Eli's breathing before he shoots his arrow it is very clear and loud because all of the other sounds have been removed to keep the focus on Eli. It shows that although he is about to shoot at something he is not nervous because his breath is very normal and not fast. This shows that he is comfortable with an arrow and comfortable with hunting or killing things.

This whole use of sound ends up being an extremely big clue for the ending of this movie because it all draws attention to the sounds of the opening rather than the visual, this is a clue because Eli is actually blind and only relies on sound to live, so it means that we are partly experiencing the first scene from Eli's perspective. This is especially noticed when Eli eventually shoots his arrow when everything is completely silent and then we hear, very loudly, the arrow moving through the air and finally right into the cat.
It still feels quite eerie in the scene though because even though there are sounds, we would expect more in a forest for example birds chirping, wind rustling, animals moving. Since there is none of this it puts doubt in the viewers mind that this is a real forest.