How do you come up with the ideas for your characters? Do you sketch them ahead of time? Start doodling in the software? What's your technique?
It varies a lot. Often I will have sketches on paper before I jump into ZBrush. I've always got some paper and a pencil at hand; it's a bit harder to bring your computer everywhere! Personally I find that doing something on paper first might help give the sculpts a bit of a twist that they wouldn't have if I started out in 3D. Especially when I'm designing very cartooney characters, I feel that I can really play with some weird shapes that I might not have come up with had I started in 3D.
I don't know where the ideas come from. I usually just let my pencil/tablet do whatever it wants. (Yeah, I've got magic pencils.) When I start in ZBrush, I start out with ZSpheres to create a very basic mesh and then just start pulling the mesh around at the lowest subdivision until I've got a great silhouette. From there I just take it one subdivision at a time. If you've got a cool silhouette the rest will come very naturally.
It's never easy to start from nothing, so when sketching on paper I've started to just make a bunch of random lines and curves until I eventually see something I can use. I used to always start with maybe the eye or the mouth, but when I start from randomness I find that better shapes pop out. I also get shapes that I wouldn't have thought of otherwise. There is probably a bit more to it than that but I wouldn't know how to put words to it.
When I sketch in 3D its basically the same thing. I just push and pull the mesh at low resolution until I get a shape that I like. I think this way works for me because it forces me to just focus 100% on the silhouette to begin with, which I feel is really what is the most important part of a character. Obviously I'm a bit limited when I set out with an idea of what I want to make (say, a T-Rex) but the technique still works and helps get some less obvious shapes into the character or creature I am creating, even if it might be something that people have seen a million times like a cat or crocodile or whatever.
Once you have a concept, what is your process for developing it?
When I'm doing my personal stuff I start out on paper with just a very simple sketch and then do the development in ZBrush. What is really important to me is to have an idea of what I want the character to feel like. This also means that once I go to 3D I might end up with something that looks quite different from my initial sketch but as long as it feels the same, I'm happy. Ever since I started using ZBrush I haven't done any side and front view sketches (unless my initial sketch just happens to be a side or front view). I'd rather spend time perfecting the design in 3D. To me it feels rather weird to spend to much time figuring out how a character is going look from all angles on paper when I can just jump straight to 3D and figure it out in ZBrush. From my experience, no matter how long you spend on the side and front view sketches, changes will always happen when you bring a 2D sketch to 3D. Personally I prefer to just go there as fast as possible.
Once I start working in ZBrush, I let myself be very open to changes and try not to stick to much to the concept, but rather go for what works in 3D. I've got to add to that there are exceptions. Sometimes I find it extremely fun to push the 3D as much as possible to make it look like the drawing. I really love it when something that somebody might say could never work in 3D somehow turns out really well anyway, like the characters in Psychonauts. (I'm especially thinking about a character called "the milk man".) I think designs like these stand out because they feel like they are still drawings that just happened to be 3D. It's something that I would really love to bring more of into my own work.