“This film was an incredible experience because we’re in one of my favorite time periods,” Butler, who plays Manson murderer Tex Watson, tells PEOPLE. “We just had such a fun time. I got to wear my own cowboy boots and I also got to ride a lot of horses in this film.”
“You already knew how to ride?” interrupted Robbie, 29, who plays Sharon Tate in the film.
“My grandpa actually was a cowboy,” responded Butler, who arrived at the event with longtime girlfriend Vanessa Hudgens. “He wears a cowboy hat every day and cowboy boots every day so he’s gonna be really proud, I think.”
Though the 27-year-old grew up in Southern California, he said he would horse-ride alongside his grandfather in Arizona — and admits he’s only been to Texas once.
“He would throw me on a horse when I was quite little,” he reminisced. “Then I went years without riding because he got kind of older and he stopped doing that.”
Butler says he got to ride horses again while filming MTV’s The Shannara Chronicles in New Zealand — but learning to ride again was “trial by fire.”
“They taught me how to ride properly with spurs on me, they would teach you for 45 minutes, a couple times a week,” he says. “And then you get on set and they throw spurs on you and stick you on a horse. It almost feels wild because you’re you’ve been working within the confines of a pen and then now you’re riding in the New Zealand countryside as fast as you can.”
“You were insanely good horse-riding,” Robbie said about his horseback-riding on Once Upon a Time in Hollywood. “Do you remember we were like screaming watching it?”
“I had butterflies the whole time. It was crazy,” Butler said.
Hyping up her boyfriend, Hudgens, 30, shared a photo of Butler sporting a cowboy outfit in the film on her Instagram Story.
“Damn he fine,” she captioned it, tagging Butler.
The dinner Tuesday, presented by Levi’s and hosted by Phillips and Carineh Martin, who co-founded RAD (Red Carpet Advocacy), invited Butler and Robbie to celebrate the vintage denim featured in Once Upon a Time in Hollywood.
Phillips gave the collaboration a charitable twist with her organization to advocate for Youngcare, an Australian non-profit that aids young people with high care needs.
She said the idea for RAD stemmed from “wanting to do good, but also still wanting to be a costume designer,” before speaking to Robbie.
“I’ve had a few personal connections with people close to me becoming disabled in their late teens and early 20s,” Robbie said about the charity in her native Australia. “My mom works for kids with disabilities so I’ve been a part of that world my whole life as well.”
“My desire was to come together around our movie that we’re all so excited about,” Phillips added. “And here we are talking about the movie that we’re so proud of, and doing some good along the way.”
At the event, Butler sported an all-black denim Levi’s outfit, already channeling his upcoming role as Elvis Presley in the upcoming Baz Luhrmann-directed biopic.
“I am just profoundly honored that Baz has invited me on this journey with him,” he told Entertainment Tonight at the Once Upon a Time in Hollywood premiere. “It’s an extraordinary privilege. And I just feel so blessed to be working with such singular directors like him and Quentin Tarantino.”
Once Upon a Time in Hollywood opens Friday.