The 26-year-old actor was in awe of his veteran co-stars - including Stanley Tucci, Susan Sarandon and Nick Nolte - but admits Redford, who also directed the political thriller, made the working environment welcoming.
He tells Entertainment Tonight, "Well, Robert sort of held my hand. If I didn't have him in my corner I'd have been quite intimidated. Because I'm still a fan at the end of the day. You know, Stanley Tucci, he's not 'Stan', you know what I'm saying? Ever.
"Even when they're friendly to you, I have an extreme reverence and respect for everybody on that list. Sitting with Brendan Gleeson (when the) night before I'm watching Braveheart... It's really tough for that to disappear and for you to be able to get in the moment. It's tough to forget all that.
"Bob's like my pal. But with everybody else, I kept (a) distance. I didn't get friendly with many people here. I was friendly with Bob in a director-actor way, but even when me and Bob would have scenes, there was distance."