Fonda, 74, had no idea how influential the inaugural music and arts event was to the hippie movement in 1969, so Keener took it upon herself to school the Barbarella star about the alternative lifestyle.
In an interview on U.S. breakfast show Today, Fonda said, "I play a hippie. People think that's what I was but I never was a hippie and (Keener) was far more a hippie than I. She told me what music to listen to and (in the film) I had to say something to (co-star) Jeffrey Dean Morgan about how I was at the Woodstock festival and (her character) breached, the baby breached when... Jimi Hendrix played The Star-Spangled Banner."
And Fonda only fully understood the gravity of rock icon Hendrix's performance after watching 1970's Woodstock, which documented the legendary festival on film.
She added, "She (Keener) brought me the documentary so I could see what that (performance was like). No wonder my (character's) water broke! It was an amazing experience. So she helped me understand what that was all about."