A client I've been working with recently asked me to create a rough mockup of an iPhone game he plans to develop in the near future. I was instructed to include the character, falling objects, a background, and some UI elements.
The basic gameplay is that you control a bacterium that has to devour falling objects to raise his health bar (shown on the right) and grow in size. Tapping on the arrows on either side of the screen controls the character's movement. Reaching at least one star allows one to progress to the next stage, but for the sake of replayability and other possible bonuses the player should strive for filling the bar completely and attaining 3 stars. The rank is based on the status of the health bar when time runs out.
The client really wanted a focus on character development, and emphasized that the character should be unique but memorable. I went through a few different designs before settling on the cube shape.
As for the falling objects, certain ones when devoured affect the player in different ways. For instance I thought sharp objects like the thumbtack might lower your health bar, while others like the sugar cube could give a temporary speed boost. I wanted to emphasize how small the protagonist was through the objects, so small items like coins and buttons could appear very large by comparison.
The background was mostly inspired by ordinary home interiors, I wanted the setting to be somewhere the character could naturally encounter small objects falling from above for whatever reason. It's mixed with a somewhat cut-and-paste paper look with the drop shadows and bright colors.
As always, if anything about this process or style appeals to you, drop me an e-mail (firstname.lastname@example.org) and I'd be happy to work with you.