All art is autobiographical. The pearl is the oyster's autobiography.
In each of my characters there is a little of me. Not strictly autobiographical but a little piece of my soul.
The task of writing a first novel* is a daunting one. I will write more later about some of the strategies I used to tackle it, but one of the things I tried to keep in mind was the ubiquitous advice that writers give: write what you know. So I based the story in the town where I was living at the time, drove by houses to observe the features I was interested in, watched people in the neighborhoods I was writing about, and took plenty of notes on all of it. I did a lot of people watching. I took photographs of events that I thought could be important. I journalled certain elements of conversations I had. I marked down plot elements and special occasions in a calendar devoted to the novel to help me keep track of the timeline. I threw in some fragments of memories from my own childhood. Then I worked very hard to fictionalize all of it.
So while certain elements may feel vaguely familiar to someone who lived in the area during the time period the novel is set, everything is a patchwork and nothing is meant to accurately represent anything that actually happened. I've seen a number of quotes about how all art is autobiographical and in a sense that is true. Obviously all art is created based on our own experiences, the things we've seen, read, heard and took to heart. As mentioned, I based many of the characters on fragments of people that I observed around me. But just because art is autobiographical doesn't mean that it has to be an autobiography. I've found that there has to be something in a character that I can relate to and resonate with as well as things that are foreign to me.
It was a fun exercise to filter all these fragments through the characters that I had created. For example, at one point a character is remembering a childhood outing to collect beer bottles from the ditches at the side of the road. This is based on a memory of my own, but specific details had to be changed to fit the story and characteristics of the person at hand. I took some of the details of the muck and slime of beer bottle hunting and extracted it from a young boy whose prime concern was how much money he'd be able to cash these bottles in for. Then I wove them into a storyline with a young girl who was thinking different thoughts and going through different life experiences. Based on a memory of mine, but a very different story.
All that to say, each character is a conglomerate of characteristics from a lot of people. Part of the hope in that is that the character will end up unique, but also broadly believable and recognizable as people will be able to see in the characters certain elements of themselves or people that they know. (My wife was reading the manuscript the other day and she set it down in frustration, saying that she was angry at both of the main characters -- especially one who exhibited negative characteristics she has observed in a number of friends over the years. I've decided to take that as a compliment.)
*I consider this my first novel--I am not counting the novella I wrote in middle school about the raindrop shaped car (which is the most aerodynamic shape, as we had recently learned). Nor will I count, from a few years later, the Tolkienesque fantasy fiction novel for which I filled notebook upon notebook with research about runes, medieval weaponry, castles, and the like. I also developed extensive maps and topography complete with notations about the locations and populations, but I don't think I ever wrote any words for the novel itself.