I think the same goes for writing to someone. If I have an issue with you, I probably won't talk to you directly (although I'm getting better!). Instead, I'll write articulate letters that I'll never send, and if you try to talk to me, you'll be stuck with inarticulate ramblings about how I don't know what I want. It's somewhat obvious how writing about a real person turns them into a fictional character, but maybe less obvious how writing to someone makes them into a fictional character. Or maybe it's obvious to everyone, but the point is that when you write to someone, you're not having a conversation. Hopefully, they'll respond, and you'll respond, and you'll go back and forth, but in the act of writing, you are the only one there. You get to put all your thoughts and feelings down without giving them a chance to respond in a way that could change the course of the conversation. You are not talking to the real person, but a version of what you hope the real person would be. You can anticipate what they say, and maybe that's exactly what they will say, but who knows? You create the version of the person that you want to talk to when you write to them.
Can true identity ever come from writing? Even in an autobiography, you write a fictionalized portrayal of yourself; how you see yourself or want to see yourself may not be how others see you or how you know you are. Is there such a thing as true identity? Who owns it? Am I being ruined by my postmodernism class? What do you think? Write me a letter, Box 3071. I promise I'll respond.