When to use init(), preprocess() and prerender()

In this thread, Craig suggests asking yourself the following questions:

1. Do I need the results of this computation in order to process a form submit?
2. Do I need the results of this computation in order to render the page?
   Then, based on your answers,

• Use init() for (1) and (2)
• Use preprocess() for (1) only
• Use prerender() for (2) only