For the first example, I’ll use this image, where the background is mostly an ambiance.

I wanted to give the impression of the soldier being in a dark environment after a fight, so I used different particles to simulate the ambiance after the fight.

To add some depth to the image, you can add different layers of particles and elements like smoke in front and behind your subject.

As you can see I have a layer of flames in front of the character, in color dodge blending mode, to add some warm tones in the air, as well as orange particles even more in front, to go with this. These particles are duplicated, blurred and then set to color dodge as well to add just a bit of pop.

 

In the background I have a layer of flames and smoke, as well as particles, but these layers are colored by a gradient map to be in blueish tones. This way the armor is also tinted with this blue light, and it helps bring together the whole image, with finally a flare of light coming from the visor.

At last, I used a final layer on top of the character and the flame because I thought there was too much orange at the bottom of the picture. By using this layer in color mode and painting with a neutral color, white, black gray whatever as long as there is no saturation, I can take out the saturation from the layers beneath. This way I paint roughly with a big airbrush, and can also use the layer opacity to reduce the effect and let a bit of color through. This keeps the attention in the center on the image where I want it.

Another tip used in a different setting, is adding bokeh and or atmospheric perspective to your illustration. The bokeh simulates the picture being taken with a camera and the focus being only on the character, and background being out of focus.

To do this, you only need to draw everything element of the background you want to blur on another layer, and then add some gaussian blur.

One thing to know is about how depth of field works, the furthest something is to what in focus and sharp, the more it will be blurred. So In this case I have only two planes, the first building and character are sharp, and the building behind is blurred. But if I had a third building even further, I would accentuate the blur on it.

 

Then comes the atmospheric perspective, just like before where the further an object is, the blurrier it is, this time it is about its value range. The furthest object has a reduced value range, especially in the dark tones. You can achieve this by having a level adjustment layer and reducing the black output. This nothing can be pure black and the whole background loses in contrast, this also helps keep the attention of the view on the character in the foreground, being the most contrast point in the image.

And the last tip is used in another image, and is to think about color harmony in your painting if you want to make your character stand out. Right here, I chose to do all the background in a complementary color to the animal to make him pop out, and I also used the blur technique to reduce the details in the background. This way by having really saturated and bright spots on the character, it stands out from the complementary background, with fewer details and contrast, not bringing the eye but just adding a nice scene to the whole image.