Johnny Depp owned the club where River Phoenix overdosed and died. He knew Phoenix had a drug problem and yet let him score inside his club. I think Depp is a good actor but a bit of a douchebag.
Phoenix was an addict and he was a spoiled brat with no sense of security.
He had been pal'ing around with Flea from Red Hot Chilli Peppers for quite some time and fancied himself a "musician". (What is it with so many actors being wannabe musicians?). Anyway, Pheonix had jammed with Flea and his group of musician friends a few times and began to think himself in the band.
That fateful night, Flea was going to take to the stage with his musician buddies and Pheonix wanted to join him on stage. Flea told Pheonix that he wasn't welcome to join them on stage (in a nutshell, I'm sure he found a way to try and be polite about it). Any way, Pheonix took that direction very badly and responded by going on a bender (as any child would do) and started using cocaine and drinking alcohol. (All the while embarassing Flea more and more.)
By the end of it, he had six times the fatal amount of cocaine in his system. The autopsy revealed cocaine and morphine (heroin shows up as morphine, as the body metabolizes), Valium, marijuana and ephedrine (meth).
All because he was asked to sit out a jam session he wasn't really capable of doing..
Yay.
Depp had nothing to do with it.