I don't know what it's like for other artists, but when I initially think of something to draw, once it's down on paper, it rarely looks exactly how it looked in my head. It's sometimes better, sometimes worse, it averages out over the years to I don't worry about it too much.
This sketch however, turned out exactly how I envisioned it. It's a simple composition but I tried working more on my values with this one, getting more 'dark' space in there than usual and getting some rim lighting going. This was sketched in it's entirety while travelling on a train to and from work. I find the rocking of train helps randomise the shading ;)