Creating a painting that has depth makes the work more real and lifelike. Without depth you do not feel the subject matter is real or lifelike. There are specific things to do to create depth, here are some I use.
First, when choosing subject matter and compositing I think of depth. I always choose to make the foreground part of the image a slightly bit bigger. If I am making a painting with flowers, I also choose one flower that will be larger than the other flowers in the portrait. If there are other flowers I make them smaller and off center. If there are leaves or twigs I think about where I place them so that the one foreground flower stands out to my eye.
When I layout the drawing I usually create some similar items that I can place in the picture that become smaller and smaller as they recede in to the background.
For this particular work of flowers, I would make the foreground main flower a little warmer in color. Then I would make the background images a little cooler in tones. Like a big flower that is bright yellow in the foreground, and for some leaves or twigs behind it, I would make them a dark cool green or brown color.
When I want to make the foreground image stand out, I use harder edges around the image. Like, I would put some brighter white around my yellow flower, but I would make the lines thick. To further make the background feel in the back, I would make sure I made leaves and twigs with very fine lines and edges. In the deep background I would try to make the colors blend together completely so you can not define any edges at all.
Overlapping elements also add depth to a work. Perhaps, I would create one big leaf that is connected to my flower that just slightly bends in front of it.
When items in the background get further away, I make them more blurred with little detail and I use faded tones of color. Sometimes colors are even hard to distinguish if the item is so far in the background it is very small and faded.
My main focus in painting and depth are light and dark colors.
1. When items are in the foreground, and need to appear closer I make the colors brighter and lighter.
2. When items are darker in color they appear to be further away.
Creating dark shading around images or darker gray shading also makes images feel like they are further in the background, as long as you use fine or fine lines. Creating white shading around images makes them stand out and appear larger.
Glazing is another tool to create depth. If you create images with layers of glazes, each layer of glaze adds a layer of depth to the painting. I Apply thin layers of under painting at first. Outline the basic painting with light colors, fill in the basic colors, then I finish off all the details in the painting using colored glazes.
I think all these steps make my paintings seem much more lifelike and 3D. I think this also gives the painting more emotional detail.