Basing characters off of real people is something I can say with some degree of confidence that every writer's done it. Some may find this unethical, but I have to disagree. A writer's goal should be to deliver some message of truth about the world, about people and a writer has to base that in their knowledge of those things. A book I once read said that writers have two places to learn from: books and life. Books can help you learn about things you can never completely experience yourself (being a different race or gender, living at a different time, etc) but life can help you create your own stories to tell. If you base all of your writings entirely off things you've read then it might be difficult to create a sense of originiality and honesty to your stories.
I base most of my characters off of real people, I've never completely ripped someone off, making their deepest confessions told to me in confidence the subject of my story, thinly veiled by changed names and slight details. That would seem unethical to me. Many characters, though, are composites of several people: habits, appearances, aspirations that I find interesting in people I've known for years or strangers I see at the supermarket. Basing them in something real makes them feel more real to me. Sometimes even bits of myself find their way into characters.
This meditation started when tonight I was about to send an email to someone and I almost addressed it to the character I had based off them. That would have been awkward to explain. My one worry is that if any of this ever gets published that people I know will see bits of themselves in my story. Especially characters that are pulled from several different people- they might be offended by the way a character they identify themselves with diverges from them more than they are by the ways the character is similar.
It's been a long day. Any thoughts?