Given many characters in a typeface build on others and even just a couple of characters can reveal a lot of the features of a given typeface, what characters are good to start with when designing a typeface?
I got interested in the question (I don't design type, I just design with it), asked around folks that do, and did some research. There doesn't seem to be a consensus -- every designer works with his/her own natural creative process, and many start with a sketched idea that could be any letter or a combination.
Here are some interviews from ilovetypography.com (an excellent resource, btw) that will give you an idea of the diversity of approach: Ludwig Ubele, Nikola Djurek and Alice Savoie. Ubele says, in particular:
The nicest part is to start: sketching randomly, finding an idea and a general construction or characteristic; drawing the first letters and making the first words. As I said before I donâ€™t have a specific letter which I usually start with, but there are some key glyphs which show the basic forms: n, b, o, v for instance for lowercase, A, H, O for uppercase.
I try in the begining not to concentrate too much on single letters but work on the whole alphabet and balance the single letters in relation to each other. That way I can set text very early on, and see how the typeface looks in small printed textâ€”thatâ€™s usually very different from what you see on screen.
The best typographic resource I know of on the web is typophile.com. There is a terrific "How To" section in the wiki, and you can branch out from there. This site will take you as deep as you want to go into typeface design.