There are many "pencil" brushes for Photoshop, both shipped with the software and distributed over the web. None (that I have tried) show the specific behavior of a real pencil.
A real pencil of about medium hardness, if you brush it over the paper only very lightly, creates a grey line – no matter how often you draw over the same part of the paper! If you hold the pencil lightly and keep going over the same part of the paper over and over again, you will create a silvery, light grey area. You will never achieve black.
At the same time, when you press the same pencil harder, your can create a dark line. How dark the line gets, depends on the hardness of the pencil, but always a light pressure does not result in dark color, even if you keep going over the same part of the paper forever.
The reason for this is that if you press lightly, the graphite will never be rubbed into the fine "valleys" of the paper structure. With a light pressure, you will always only color the "tips" of the paper grain. If you want to color the whole paper, you need to press and "level" the paper grain (or rub the graphite into the "valleys").
In Photoshop, on the other hand, all "pencil" brushes lead to a black area or line when you keep drawing over the same area for some time.
I have tried fiddling with the brush settings for quite some time now, but I don't know how to emulate this specific effect of a real pencil on paper. There is the background-foreground jitter, but that will create a light grey line on a dark area, if I go with light pressure over a darker color, which is something a real pencil wouldn't do. A real pencil does not lighten dark pencil lines when you draw lightly over darker areas.
So how can I emulate the effect I describe in Photoshop?
I have Photoshop CS5.1, but would even be willing to upgrade to a current subscription-based version or change to a different software, if what I want is possible there.