I don't agree with writers who say characters are the only way to start a project. They say plot comes from characters and only from characters. You should know your character almost better than you know yourself before you start writing. You should know every bit of detail about them, from who their first crush was to what the first solid food they ever ate was. Only after you know all about these characters can you even think about what the plot of the story will be.
BS I say.
Don't get me wrong, having a good character, one who the reader can relate to, is extremely important to a good story, but in my opinion, plot is king. The character is there to help tell the story, to entertain. The character should fit into the story, not the other way around.
Of course, when the story is finished, that shouldn't show. It should look like the story evolved because of the characters. The way this happens, in my opinion, is not because of intense characterization before the process, but because the author learned more about his or her characters in both the first draft and the revision process. The author should think about the characters a bit before writing starts, but how much time spent depends on the project.
Next time I'll discuss my thoughts on that last sentence in more detail. Until then, have fun reading and/or writing!