Sometimes I like to contemplate the process by which someone's interesting life turns into a very interesting novel. It is definitely not a straight line from Point A to Point B. In order to turn into a novel, the material has to sit for a while, like a good wine that will be aged in casks of mellow oak. Two strange things have to occur. As more flavors enter into good wines when they are stored, more lives have to enter the mix. Then something semi-mythic has to frame them. It is as if they have to ascend to a higher register, the essence of what they are, but now fictitious. Again, like wine when it acquires its defining note. The truths they tell have to become larger, stranger, sometimes menacing and threatening, with a dangerous beauty. And I am not talking about the characters, who may or may not be menacing or strange. The character has to transform somehow before it reaches fictional maturity, which is not at all the same as being mature as a human being. Fictional maturity occurs when a character achieves an inward consistency, which means that her stamp is present in the way she acts and does and is in all of the novel. "Fully realized" is another, older way of saying this.
Conversely, as hard as you try to write someone's life and think you are being "faithful" to its events and occurrences, it becomes fiction the minute the words hit the page. Somehow, in just trying to write what someone is about, you enter the realm of fiction because you are writing at a remove of years, and there are many many reasons why someone may or may not have done something, worn something, said something, thought something. You are in essence making choices at each step of the way in portraying someone. And if you write someone and make certain choices, you might make completely different choices in writing him the next day.
This is also why most "real" accounts of something or someone are a kind of fiction, as well.