The main character is not the main character
I usually start by making a lexicon, and it's just that, a file named “lexicon.txt”. This is where I jot down notes and ideas about my story. Usually a paragraph with the basic idea, and then as I go I add names of characters and little tidbits of information about them, for me to refer back to later on.
My main character was just the name, with a description of “main character”. I have a major supporting character that I also started writing up notes on, and then kept writing more notes. I had a lot of ideas for this character, and didn't intend for the main character to meet this character right away.
I started writing, and got about fifteen pages in before I realized that I was writing mostly about this supporting character. More and more ideas about her backstory, and where she lived, kept coming, and I'd written almost nothing about the main character. It became obvious that the main character is not the main character, this supporting character is the main character. So I backed up to the beginning, and started over again.
Once again, I got a few pages in, and then I realized I was starting in the wrong place. It was the middle of the story, or maybe a little more toward the end, and there was a lot of backstory to cover. So, I started over again.
I wrote 1,832 words today.