The older brother of Joaquin Phoenix died aged 23, after suffering drug-induced heart failure outside The Viper Room club in Hollywood on 31 October, 1993.
Twenty years later, Richert has shared his memories of his A Night in the Life of Jimmy Reardon star for People magazine, revealing the wild actor was a "risk-taker", but was never hooked on drugs.
He tells the publication, "This idea that there was a drug decline in his last year is erroneous: River was never a junkie. The only instinct River didn't have was for survival. He never would believe that someone could give him something that could kill him."
Actor Corey Feldman has also opened up about his relationship with Phoenix after co-starring in 1986 comedy Stand By Me, and he admits he tried to reach out to the star as his personal battles became public knowledge.
He says, "I was not aware of his addiction issues until shortly before his death. (Someone) told me... he had a heroin addiction. Everyone knew that had been the demon I had battled. I didn't believe her at first. He seemed like a healthy guy, always doing the right thing. I told him I was sober and that I was going to Aa (Alcoholics Anonymous) meetings and could maybe take him to one. He said, 'That sounds nice.' I called him back several times, but for whatever reason, we never reconnected."
Filmmaker Peter Bodganovich, who worked with Phoenix on 1993's The Thing Called Love, Running on Empty actor Judd Hirsch and director Joe Dante have also offered memories of Phoenix for the anniversary piece.