Prominence plays a role too. Even if there were 100,000 people named Stephen King, they wouldn't use it for a fictional character.If memory serves, there have to be either none, or more than a certain number of people with that name before you're allowed to use it. The number is quite low if memory serves.
The American release of Godzilla comes to mind. The original movie from the '50s had Raymond Burr in it as Steve Martin. When Godzilla '85 rolled around, they just referred to him as Doctor Martin. (Or maybe Mister Martin, can't recall for sure.)