The "Thelma and Louise" actress, 66, said Susan, 76, learned how to be "genuine" from her. They both appeared in the 1991 movie.
On Monday, October 22nd, Geena discussed her former co-star with "Good Morning Britain," saying, "It really transformed my life, because somehow in my sheltered existence, I had never spent time with a lady who speaks what she believes without qualifiers in front of it.
"I was busy dying of politeness; I was used to live as someone who says, 'I don't know if you agree with this or if this is a good idea.
"Oh my God, this was a totally different universe as soon as I met Susan. Nobody had reacted differently to her, either. She was beloved by everyone.
"Wow, you can really be like that, I thought.
She demonstrated for me what it's like to live honestly and in the present.
Geena made the statement while describing research her Institute on Gender in Media has conducted showing that women continue to be marginalised in the film industry.
"My institute conducts all the research," she continued, "and current statistics indicate that 20% of movie characters are 50 or older but just 5% are female, so it's such a little percentage of the roles we get."
As Brad Pitt won the 1990s part of thieving grifter JD in "Thelma and Louise," George Clooney jokingly told Geena that he "hated" the actor.
According to the mother of three, George Clooney once said, "I detest that Brad Pitt, he got the job in 'Thelma and Louise' over me. I was sitting next to him on a flight."
A spot on Brad, 58,'s bum was covered up by a makeup artist while they were filming their iconic sex scene, according to Geena's new novel.