"Based on a true story" is a come-on, the aesthetic equivalent of "no loan request refused." For, at best, the creator has fashioned a film based on his understanding of, interpretation of, and reduction of the report of an actual occurrence.
Hitchcock denigrated American films, saying they were all 'pictures of people talking' - as, indeed, most of them are.
Film is a collaborative business: bend over.
Films have degenerated to their original operation as carnival amusement - they offer not drama but thrills.