The Oscars favourite admits he has the same discomfort on movie sets as he did when he was studying among the elite at Oxford, revealing it was a culture shock when he first arrived.
"I still sometimes find myself confronted with that discomfort," he told the BBC's "Grounded With Louis Theroux" podcast. "You know, that sense of imposter syndrome if you're not to the manor born."
"My parents were just very focused on education. Rather than going on holidays or anything like that we (would) try and get a tutor in to get us ready for the entrance exams, try to work out how to get us into private schools which were, in many ways, like a culture shock to begin with, just like Oxford was."
He added, "In most rooms, where decisions are made, it's about being able to be conversant in that upper middle class English, and I don't just mean in terms of how you talk, but just being comfortable in those spaces, which took me a long time."
"The film industry in the U.K., I think, is rife with the same kind of classism and it was a weird experience."
Riz is glad he didn't quit his studies at Oxford even though he felt "very isolated and alienated from the whole vibe there."
"Then I thought, 'Actually, the place where you don't feel like you belong is maybe where you belong, is where you should be, is maybe where you can contribute something new, where you can grow'," he added.
"We have to learn to be comfortable with the discomfort of that. And I ended up having a great experience, where I did a lot of acting in the plays there, and I put on a club night that helped pay my way through that... It's usually the way that the challenges are the gifts."