My source for this answer is "A Karamazov Companion" by Vitor Terras (1981), University of Wisconsin Press.
The Brothers Karamazov contains more autobiographic elements than most of Dostoevsky’s works (...)
Let's go through some of the characters.
It seems that Aliosha was based on people from real life.
O'Brien is the party, he is the one who creates the reality for others. He is a member of the "Inner Party" and the one who creates the illusion; O'Brien is the one in charge of everything and controlling society, and he is the one that labels thought criminals so.
I haven't found any evidence, that any of the characters are taken from the real life. Researchers point some character inspirations from the real life but these are just theories. And yes, Dostoevsky did build and develop the characters while creating the novel. Russian's Wikipedia featured article on the topic is rather comprehensive, I don't know about ...