Just as an informal, nonscientific observation, most people's personalities don't seem to change very much during their lives. There are exceptions in the case of people who go through hugely traumatic events or suffer from brain injury or disease. Some would argue that religious conversions can have deep personality-altering effects. But these are all exceptions to the rule.
Neal StephensonBut I'm not a small-literary-novel kind of guy, and once I'd developed the world in the first couple of hundred pages, I felt that there was potential here to go on and write an engaging story set in that world. So that's what I did. This probably ruins things both for the people who want small literary novels and for those who want action-packed epics, but anyway, it's what I wrote.
Neal StephensonThe less attractive the character, the more I enjoyed writing them. Officious bureaucrats and PowerPoint weasels are where it's at for me.
Neal Stephenson