I managed to get the characters animated today, along with a few smaller bits and bobs. It makes them seem a lot more alive, even though the animations themselves are relatively simple. I think randomising their idle animations helps, as tempting as it is to go for the classic 16-bit idle animations like Sonic tapping his foot!
Sound effects and mini-level introductions up next, I might post a few of the latter to see what people think of them. Basically, at the beginning of each level, either a new concept is introduced with a tutorial, or an image is shown to help set the scene or give some back story. The game itself isn't all that story focused, but I'm hoping that these scenes will help to give the player more connection to the characters. Or at least will be a mildly entertaining way to start the level.