how much creative latitude did Capote make as well?
IMO, a lot. I read a book by Ralph Voss "Truman Capote and the Legacy of 'In Cold Blood'" and he goes into this a lot. Voss said that Perry's statement to the police showed grammatical errors that you wouldn't expect if Perry had been a grammar stickler as Capote presented him.
Did Perry really utter those famous words?
I take it you refer to, "I didnít want to harm the man. I thought he was a very nice gentleman. Soft spoken. I thought so right up to the moment I cut his throat."
I don't believe he did. Perhaps something vaguely similar. I recall in the movie Capote playing around with the words until he came up with the above.