Terrific doco, it's called "I Knew It Was You: Rediscovering John Cazale".
http://www.imdb.com/title/tt1352717/Available from The Usual Sources, last time I checked.
John Cazale must be the only actor ever to score 100% when it comes to the quality of films he's been in - a TV appearance and a short film aside, there's not even a mediocre film in the lot. Five classics out of five films.
And his long-term girlfriend was none other than Meryl Streep.
Regarding Fredo himself - part of the power from the portrayal comes from the weakness itself. Fredo's not an ubermensch like Michael, he's not a cold-blooded automaton like Tom, and he's not a cartoonish hero like Sonny. He's just some guy, caught in the middle of this mess, looking to make the best of his situation, just as anyone would.
That he would fuck up trying to make something of himself isn't surprising, and it's completely sympathetic. Vito, Michael, Tom, Sonny - gods of the Corleone Family, but almost otherwordly. Poor old Fredo? What chance did he have? Of course he got suckered in, and it's even more heartbreaking that he only ever wanted to please his father, and then, his little brother.
