I guess it depends on how the person is portrayed--if the writer is able to present the character in a fashion true to the real person--speech patterns, mannerisms, theology, etc.
It bothers me when an author is writing historical fiction, and places modern-day thinking on the character. Sure, people are the same throughout the ages, but really, they were less concerned about the environment 3000 years ago! If you know what I mean.
A recent novel I read stated that the girl would be free from someone's guardianship when she reached 18. The problem was that the book was set in the 1870s and legal adulthood in those days was only granted at the age of 21. A very small matter, in and of itself; it demonstrates how easy it is to make mistakes of that kind.
Pattertwigs Pal wrote:If I remember correctly, Lewis also used himself in a fictional role in one of the Space Trilogy.
Yes, in Perelandra. I just read the trilogy for the first time a few weeks back, and noticed that. I found that it jarred on me. He did the same thing in the Chronicles, but for some reason it feels different. More subtle, perhaps.