What are the key disciplines of realistic drawing?
Definition of drawing

Drawing is the technique of putting marks on a surface to create an illusion of the thing or things to be represented. Drawing can be done with pencil and paper, digital drawing tablet or even a piece of silver wire on gessoed panel. 


  It sounds pedantic, but one of the fundamentals of drawing is remembering that a
  drawing is a representation of a thing and not the thing itself.




Emphasize the details

A well-crafted drawing emphasizes the features of the thing being drawn, those features that define it. This emphasis is made by the juxtaposition of a range of values (shades) and sometimes colors (tints). It can, for example, be accomplished with shading, cross-hatching, stippling and/or variable-width line-work. Indeed, the tool(s) being used will give the drawing specific characteristics that can be aesthetically interpreted.


  An ink contour drawing of a banana can be executed in several
  well-placed lines, whereas a shaded pencil drawing of the same subject
  can literally be composed of thousands of marks of the same intensity layered upon one another to “build up form”.




Seeing is forgetting

When learning the skill of drawing, it helps to compare details, like the distance between the eyes of the model’s face as related to the width of the mouth, and how those distances relate to the diagonal distance from the outer corner of the right eye, the left nostril and the point where the upper and lower lips meet in the left hand corner of the mouth. Then a comparison is made between the real face and the drawing of the face, adjustments are made and recompared. Then, the next comparison is made between the lips and the ears, for example and the process continues.


  Even if you are drawing a tattered zombie with a tree stump as a club on a  battlefield surrounded by forest elf cavalry mounted upon giant mutant
  frogs, things like proportion, perspective and shading are still
  necessary skills to have mastered before the drawing could be considered realistic. However, if you have no idea how frogs
  should be saddled, you should get some visual resources (like frog
  pictures and saddles) and use drawing to figure out how it should look. Sketching is one way of finding answers.




Start generic and generate detail

In many classical life drawing courses there is a warm-up phase, where the model changes the pose every several minutes. There is simply not enough time to do a completely detailed drawing, so these warmups make the artist look at the whole subject to capture the feeling of the pose, the emotion of the body language. This is like sketching, trying to “figure” out the body. Then there are somewhat longer poses, perhaps 5-10 minutes. In which details can be focused upon. And then after a break, there might be one or two longer poses in which “finished” drawings (perhaps including backgrounds etc.) can be made.


  If you take this life drawing example as a model for creating
  realistic drawings you should: first create a general volume in
  perspective space for your subjects, then pay attention to details
  that bring out the specific character and then apply finishing touches
  like shading.




Light and dark

Shadows and highlights are what make a drawing seem realistic. Here, you would do well to study the works of the old masters like da Vinci and Michelangelo. Especially the folds of fabric and the glisten of metal are hard to master but rather important if you want to succeed to make drawings of things that appear realistically drawn. 


  If you set up a still life for material studies, be sure to include a
  light source that you don’t move until each study is finished. Then
  move the light and make the same material study. The casting of
  shadows and reflections of edges within shadows are phenomena that you
  should pay attention to.




Colors change with different neighbors

If you only have one color, you are making a monochrome drawing, which can also be interesting and realistic. The thing about color is that it is really quite challenging to learn about palettes, warm colors versus cold colors, etc. 


  This is one place where every teacher will have their own opinion,
  which is good and should remind you that the feeling of a
  drawing can really be defined by the colors you choose. A good place
  to begin is to choose a highlight color (i.e. the color temperature of
  the light source), a shadow color (i.e. the basic color of the material minus the highlight color) and a midrange color (the basic material value of what
  you are depicting) and experiment with shades of each. This is also one of
  the bridges from drawing to painting, but I’ll ignore that for the
  time being...)




A few tips


The darkest black is next to the brightest white. (Contrast pushes adjacent values further than their respective absolute values.)
Use black sparingly, if it all. It is better to have a deeply dark and vibrant color than mere black.
Treat materials  that you are depicting uniquely. (Don’t use the same shading technique on both metal and skin, for example.)
Don’t draw people or objects from photos, use real things and people. You will find that understanding the volumetrics make your drawings more than just mere copies.
Do draw architecture and landscapes from photos, because this will help your understanding of perspective.
Find a community. (This really helps you generate feedback, see your work in a new light and push yourself to keep working.)
Learn to recognize the point where the drawing is good and have the courage to stop.
Once you have good technique you may begin to break the rules.






*seeing is forgetting the name of the thing one sees is a great book about Robert Irwin that I totally recommend. It is not about drawing per sé, but a book I really enjoyed reading when I was in Art School and gives great insight into one of the most formidable artists of our time.