Another practicality is that the canvas does not chew memory; at such a miniscule size your brushstrokes can be as free and wild and quick as you like, with no danger of lag. Of course, with today's increasingly fast computers this tends to be less and less of an issue, but I still like to start off at a small scale so as to resist the temptation of jumping into the details immediately.
As the name would suggest, thumbnail sketches (or simply thumbnails) are very small scribbles, designed to be fast, putting down what you feel, emptying your mind of your current thoughts so that they may be replaced by more ideas, and by jotting these ideas in quick succession you are aiding the velocity of the process.
If you are wondering what sort of sketching you should use to document your ideas, or how much is too much or too little, then you're most likely not alone. The answer is that you should use as much information as you need, but as little as possible. If you feel you can sketch a character using just plain old line work, then so be it. If you need to put in some value to bring out the form, then do that. There is no single answer for everyone, and so you should document your ideas using what you feel comfortable with.
Please, don't be afraid to go crazy at this stage, every thought is fair game to be plotted on your sheet - in fact, some thumbs will simply be filler used to dirty up the page. I don't know about you, but a blank sheet staring me in the face is intimidating, it's saying to me, "I dare you to dirty me up ... Oh no, actually I double dare ya!" To which I usually reply by throwing down a few incoherent lines to get past that initial Clean Sheet Syndrome. Once that page has been violated with scribbles, it is no longer as imposing to draw on and a mental barrier is broken, allowing your sketches to flow more freely.
Trying Out Different Types of Sketches - As you can see from Fig.02, I am not coy about creating "dirty" marks on the page - in fact I think it can go some way to breaking that computer illustrated look that so many digital art beginners seem to fall into.
I tend to work with very simple brushes, or brushes that come standard with the programme I use, which is Photoshop - mostly a combination of soft airbrushes and harder edged airbrushes with reduced spacing so as to mimic continuous tone. I tend to use these brushes as a high-tech version of a pencil or a block of chalk, typically starting by laying large areas of tone onto the canvas before cutting back into the shapes with white.
In order to facilitate this quick process I mainly use my stylus, the spacebar to grab the canvas, and the Alt key to colour-pick the tones I want from previous laid down strokes. When you get used to it, this is a very quick method of working and allows you to put your ideas down very quickly.
You may also notice on the illustration (Fig.02) that there are some images that look very similar to each other - herein lies another of digital media's advantages: the ability to create variations simply by using the Marquee Tool and creating a new layer using the existing illustrated layer as the source. This will then allow you illustrate over the image, creating a variation side by side to the original. The beautiful thing is that it frees up your inhibition to experiment and can be done infinitely!
Now, up to this point, we've been thinking of the sketches as a personal tool - that is, an external representation of a myriad of internal ideas in an attempt to organise free-flowing thoughts into a structured pattern for our own personal use. We have part of the design in our minds and this can often cause us to stop short of creating sketches that mean anything to anyone but ourselves.
This situation would be fine if the work we are doing is only for ourselves, however, most often the art we do isn't just for fun, it's because someone is paying us to deliver. These people need to understand what we are thinking at every step of the process to reduce the likelihood of going in the wrong direction down the line - it saves them time (and money), and it saves you the frustration of having to do major rework.
So this is a very important consideration to keep in mind (important enough that I am reiterating it) - as commercial artists, we never operate in a vacuum, our work is generally part of a greater whole, in editorial enhancing the writing or as concept art which precedes the asset building phase of game or film development. In short, we need to share our ideas effectively with other people, and most often with people who are not artists.
Cleaning Up - Looking at Fig.03 now, I have chosen to clean up this design because I feel the character has poise, balance and potential to experiment - it is also the least developed and would demonstrate the process between a rather abstract image built of large shapes and how you would begin to add in the design elements gradually.