There is a brilliant documentary Friedkin Uncut where he is extensively interviewed which I mentioned here some years ago.Well, you and @David Kronenbourg would know his films better for sure
I mostly wanted to confirm that he was indeed a no-nonsense S.O.B.
And as Limmy would say, very funny
Friedkin's first feature - a documentary - saw its subject, an African American man on death row, cleared. Bill interviews Fritz Lang. Bill talks about music and picking Tangerine Dream or The Germs for his films. The controversy around Cruising? i.e. having a serial killer in the gay leather club scene stigmatized gay men?? Bill didn't care, he welcomed it if anything.
There is also lots of footage of a public Q and A session similar to what Deadmanposting mentioned. Great story telling and rapport with the audience.
There are many of his films I haven't seen. I'm definitely not a film buff and it takes a lot to impress me.