Martell has a tight-knit friendship with his "It" costars. He frequently shares his hangout updates with the young actors on his Instagram. "On 'It,' that was one of the first projects I did with kids my age, and it was like summer camp. It was the most amazing, unforgettable experience, and we're all best friends, and we are constantly talking. I'm very lucky for that experience," he told Jim Alexander of Reel Talker in a 2020 sit-down interview. "Within the first day, all the kids from 'It,' we felt like we had known each other for years, so I think it's a special thing."
Martell also opened up about how competitive the entertainment industry can be and how they've managed to remain friends. "We obviously go out for some of the same things, but they are all my best friends. We all go out to celebrate, we're not competing with each other — we're happy for each other constantly. I know they're talented, and there are roles for everyone. Some roles are better for other people, so when it happens, you remember everything happens for a reason," he told Rain magazine in a 2020 profile story.
