Getting The Ball Rolling

Question: When you have an idea for a book or story, do you tell people about it? How do you start writing? Do you think about it for a while? Write notes immediately? Think about the character first? Or plot first? Outline?
The start of a project can come about in a few different ways, but usually I've put a lot of thought into the characters and the crime. A few times, I've sat at the computer with a vague idea in my head and just started typing. The opening scene to 5,000 word scene in Cold...