Creating and developing concepts

Before you can start working on anything, you need to have at least a basic idea of what it is you want to create. Do you start creating after figuring out your basic concept and develop it along the way, or do you prefer to improve and build on your concept, developing all of its aspects before you start creating?

Are your concepts often simpler, or more elaborate and detailed? Do you focus on particular aspects when you build up concepts, or does it depend on what kind of idea it is?

Any concepts you've created or seen that you think are worth sharing? Don't forget to link to the creator if you do mention others' concepts!