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First, as others have noted, real people's lives are rarely as event-filled as the lives of fictional characters. For example, Howard Carter became a world-renowned archaeologist when he discovered...
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Source: https://writers.stackexchange.com/a/34395 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
First, as others have noted, real people's lives are rarely as event-filled as the lives of fictional characters. For example, Howard Carter became a world-renowned archaeologist when he discovered the tomb of King Tut. John Champollion became famous for deciphering the Rosetta Stone. I could give other examples, of course. For each of these men, this was the achievement of a lifetime. They then spent the rest of their lives examining their great find and refining their conclusions. But for a fictional character like Indiana Jones, we expect him to make AT LEAST one discovery like this in each movie or book. A fictional archaeologist will likely make dozens of amazing discoveries in his career. Because, "and then he went back and studied the same thing further and added several footnotes to his book" would not make a very exciting story. Second, any story that uses a real person as a character is almost inevitably a lie. First of all, the person's life has to be simplified. "Docudramas" often create "composite characters". That is, the real person may have had ten good friends who all encouraged him to do whatever over the course of years. But in a story, we don't want ten characters each of whom has some small role. The writer often combines these into one character with a big role. Something that a real person figured out bit by bit over the course of many years gets simplified down to one "eureka" moment. Etc. Furthermore, many important real-life events happened when no one was watching. At least, no one who wrote it down. The writer has to invent how these things might have happened. He has to invent what was said in private conversations. Etc. We tend to expect fiction to be definitive. I mean, for everything to be wrapped up neat and tidy. Characters may be complex, but in the end we expect the hero's motives to be basically good and the villain's motives to be basically evil. I've seen many docudramas where the writers apparently found it necessary to make excuses for bad things that the hero did. The hero can have character flaws, but he must redeem himself by the end. He can run from danger and abandon his friends in scene 1, but if he does, he must be incredibly courageous by the end. He can be inconsiderate of his wife at the beginning, but by the end he must demonstrate his great love for her. Etc. It is surely POSSIBLE to write a story with a truly flawed hero. But apparently writers find this very difficult to actually do most of the time. And finally, if you use a real person, you then have to battle between your opinions about this person and the readers' opinions. If you present him as a great hero, there will be readers who think he was not such a good guy at all, maybe even that he was a horribly evil person. And vice versa.