As Hollywood continues to mourn the death of Robert Redford, Iain Armitage is looking back at his time working with the legendary actor.

Armitage shared several behind-the-scenes photos of himself and Redford working on the 2017 Netflix film Our Souls at Night via Instagram on Tuesday, September 16. “Almost a decade ago I got to work with him on a small movie filmed in Colorado Springs,” he wrote. “The whole experience felt like a dream. It’s hard to explain how important watching legends like Jane Fonda and Robert Redford was to me, but even at that age I knew it was crucial and wrote about it to him in the note I’ve posted here.”

Though the film’s crew referred to Redford as “Bob,” Armitage said he chose to call him “Mr. Robert.” The Young Sheldon star added, “My main memory, though, was that he was kind to me. I was a lucky kid who got to roast marshmallows, go fishing, play baseball and assemble toy trains with Robert Redford in a beautiful part of the world. I am grateful for it and will never forget it. ❤️❤️❤️.”

Armitage’s post included a photo of the letter he wrote to Redford while working on the film. It read, “Dear Mr. Robert, It has been a pleasure to work with you! I’ve learned so much from watching you! Sometimes when you don’t even know it, I am watching and learning…My best memory is when we played the train set together…Thank you for being so nice to me. I am so proud to know you!…Love, Iain.”

Fans appreciated Armitage’s kind words for the late actor. “So beautiful, Iain. That kindness is part of his legacy. What an amazing gift that you had that time with him. ❤️,” one person commented underneath the post. Another added, “I did not know about this film, now I need to look for it. Very sweet tribute 😊 💔.”

“I know I wouldn’t have been able to call him Bob either! How lucky you are to have gotten to work with him ❤️,” someone else shared. A different user wrote, “I watched this only last night & it’s a superb movie, then heard today he passed away. What an amazing exp to work alongside him & Jane Fonda & for them to work with you.”

Our Souls at Night follows the friendship-turned romance between widowed neighbors Louis (Redford) and Addie (Fonda). As his bond with Addie deepens, Louis also forms a sweet connection with her grandson, Jamie (Armitage).

News broke on Tuesday that Redford had died at the age of 89. The actor reportedly died in his sleep, though a cause of death has not been publicly shared.

Fonda reflected on working with Redford in an Instagram tribute of her own. “Our last film together was Our Souls at Night. Here we are with Iain Armitage (soon to become ‘Young Shelton’),” she captioned a pic of the trio from the film on Tuesday. “Bob made a real difference in all good ways. He represented an America we must now fight to protect. He revolutionized independent film making and made us swoon in so many movies.”

She continued, “I am very sad today. Cried all morning. But luckily I can think back on so many joyful, laughter-filled moments when his practical jokes would crack me up. I feel so lucky to have made one of his first big movies with him, Barefoot in the Park (I fell madly in love with him on that one) and his last (the aforementioned Souls at Night). I was wanting to go see him these last few months to make sure we were all right between us but I didn’t act on it soon enough. Lesson learned. When people are our age, late 80s, don’t wait.”

Fonda concluded her post by thanking Redford for “all the pleasure you brought over the years. RIP.”