Out of the movies about Monroe that I've seen, I think I liked the Blonde mini-series of Joyce Carol Oates book with Poppy Montgomery the most. A lot of it was fictionalized and it could've been better adapted, but I felt like she was really solid and did a good job of not making Monroe seem one-dimensional. I love at the end when you see her just starting to crack.
Norma Jean and Marilyn was an interesting experiment, but I don't think it really worked. The way Judd plays Marilyn pre-stardom is so hard-edged compared to how Sorvino interprets the role once she's becoming famous. There needed to be more of a smooth transition there. Plus, Sorvino pretty much just imitates Monroe's performance in Gentlemen Prefer Blondes throughout the rest of the movie. It still held my attention, though.
Of the other Monroe movies, I've heard good things about Insignificance where Theresa Russell plays a character that's pretty closely modeled on Marilyn. I've also heard that the tv mini-series Marilyn: The Untold Story based on Norman Mailer's book with Catherine Hicks (from 7th Heaven) is relatively well done.
Try reading William Goldman's book Tinsel about the search for a Marilyn Monroe type for a movie based on her life. It's fiction, but you Goldman's expertise on Hollywood is in every page of that book - and I doubt it's changed much in the last 20 years.
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