Season four of the hit Netflix royal drama covers the breakdown of the pair's marriage, with Charles on one occasion seen flying into a jealous rage after the Princess of Wales proves more popular than him during a royal tour of Australia, among other incidents.
The 30-year-old actor tells MailOnline it took him time, after playing the part across two seasons, "to work out what really triggered" Charles, stressing that his take is a fictionalised one, based on real events.
"He wants to marry Camilla (Parker Bowles, Prince Charles' current wife), but he's shut down," says Josh. "He goes to the Queen to say, in essence, 'This marriage isn't working. Diana's not happy, I'm not happy. Camilla's not happy.' And he's shut down, shut down, shut down."
The brooding tensions lead to a number of confrontations which, according to the actor, left him feeling "awful" for terrifying Corrin - with whom he'd developed a strong bond while working together.
"I felt awful," the young actor admits. "I tend to keep myself to myself, especially on days where it's that kind of emotional scene. Emma and I get on very well,' he told me. When they were filming the 'Australian' scenes, on location in Majorca, they 'roamed around for weeks on electric scooters.' "
"But on the day, they go, 'Action!' - and I scream in her face. All I want to do is give her a hug and say, 'I'm so sorry.' "
He goes on to explain his heightened performance is inspired by creator Peter Morgan's writing, which he describes as "Shakespearean in its melodrama." Josh also drew on Michael Corleone - Al Pacino's mafia boss character in "The Godfather - remarking, "I definitely referenced him."
"The Crown" season four debuts on Netflix on 15 November (20).