I never pretend to be the expert on writing but I do feel that after a few books, I've learned a couple things along the way. Although to be honest, basing characters on people I actually know never seemed like a good idea and therefore, I never did it. To begin with, I'm assuming there would always be a paranoia of someone 'discovering' that a character was strangely like them not to mention hurt feelings, potential arguments and maybe even a few Facebook rants...as much as we all love those.
Most importantly, I feel that characters create themselves. You might have a few ideas of who they are or what they're about but in the end, characters have a tendency to form naturally as you write the story. It's kind of like having a child and deciding on the kind of personality you think they'll have; chances are, they'll soon show you that you never had a say in the first place.
Characters are complex. As a writer, you'll probably have a deeper understanding of your characters than most people in your life. You have the special ability to see inside their heart and understand what makes them tick and it makes sense because after all, you're on a long journey together.
The most interesting thing about characters is that they often are a piece of you. Just like every songwriter inserts a piece of who they are in their songs, every author tends to do the same with their characters. It's not something you think about but rather it just happens. Maybe one of your characters shares one of your biggest fears in life or your fixation for a 90s HBO series. The characters are unique but they're also you.
And really, isn't that what makes the writing process so amazing?