Some of it is fictionalized. I don’t mean just like conversations between members of the Clutter family, but other stuff.
Most notably, the final scene of the book where Dewey runs into Susan Kidwell, now an adult, in Valley View Cemetery, never occurred. (In Cold Blood (1967) - FAQ Capote himself has admitted that he invented the final scene where Detective Dewey runs into Susan Kidwell, one of Nancy Clutter's girlfriends, in the cemetery. www.imdb.com/title/tt0061809/faq)
Alvin Dewey, who was present at the hanging of Hickock and Smith, said Perry didn't apologize for the murders.
In the book, Perry is constantly correcting Dick's grammar; in real life, his statement revealed grammatical errors which would indicate Perry wasn't such a stickler for grammar. Ralph Voss said that in the book mentioned below.
In Ralph F. Voss’s book, "Truman Capote and the Legacy of In Cold Blood", University of Alabama Press, 2011, ISBN 0817317562) he discusses how another writer (I'm thinking the name was Tompkins, but I'm not sure) had gone to KS and talked to people and found that some things weren't as detailed in the book. One that stands out in my mind is that Josephine Meier, wife of Wendell Meier, assistant sheriff at Garden City, said she rarely talked to Perry.
I think Capote did a lot of embellishing. I think he made Perry Smith out to be less cold-blooded than he was in real life.
Which doesn't take away from the book--it's still a great book.
If you're interested in ICB, I highly recommend Voss' book.
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