Ok, here is my method on how to make eyes look as real as possible, or at least have depth. The first picture is a deers eye, with no eyelashes, so you can see the shades of the eye and the reflections.
First draw out the shape of your eye with a faint pencil, I use 4H hardness.
Next i shade the area around the eye, judging where I want my light to come from. Eyelids make shadows on the iris through the lens, and also reflect off the smooth surface of the lens.
I carefully make pale shadows to bring out the natural depth of the iris.
I fill in the darkest areas
Add more shading, being careful to not over do it. Slowly i build up the shadows. notice how I have the reflection bubbles on the lens. these should show the curve of the lens, this gives the lens depth and shape.
Finally I add in the final detals of the eye, wrinklesor color variations in the iris, shadows above the cornea all applicable shadows that need to be deepened.
Heres a mountain lions eye.
Trace out the basic shape.
Begin to build your shadows. this cat has a huge reflection, notice the real picture.
Build up your shadows behind the reflection before adding any shading the reflection bubble might need.
these only took me about 20 minutes to draw between them. Quick and simple. Im sure i could do better if I spent more time on them, but these are jsut rough sketches. Reference reference reference lol as the pros say.