The third season finale of “The Crown” is all about Princess Margaret (Helena Bonham Carter) and the collapse of her toxic marriage to Antony “Tony” Armstrong-Jones (Ben Daniels). To capture the violence of their relationship and how Margaret’s joyous moments without him are poisoned by her memories of their fights, Bonham Carter and director Jessica Hobbs had to “open up to each other,” Hobbs says.

“For me it’s a very tender, poignant piece of work,” she explains. “It’s about mental health and vulnerability. No matter what position in the world you might have, you can still feel unseen and unheard and lost. She’s a very raw character; she’s somebody who lives life in quite an emotionally exposed way. That’s always a really challenging area to work in; it asks a lot of the actor-director relationship because there has to be a phenomenal amount of trust on both sides.”

The pair met during read throughs for the third season, and developed a trust very quickly. Hobbs says Bonham Carter would invite her over to her house and they would spend “the whole afternoon” discussing the scripts, research and recordings of the late princess.

Their rapport was firmly cemented by the time it came to shooting the final scene of the season between Margaret and Queen Elizabeth (Olivia Colman), which they both recall as a particularly grueling, yet rewarding eight-hour affair.

“A lot of directors will come on and be a control freak, and she isn’t, and that’s what was so exciting,” Bonham Carter says.

After Margaret’s failed suicide attempt, the Queen comes to visit her and “finally admits that she was the most important person in the world to her,” Bonham Carter says, after almost 10 full episodes of keeping her emotions below the surface.

“There was an amazing silhouette of the Queen kissing Margaret’s hand, and [showrunner Peter] Morgan loved that. And it was so interesting because in the end product you didn’t keep it,” Bonham Carter says to Hobbs. “I just thought, you’re so out for authenticity rather than style or something that looked good — you always go for the emotional truth, rather than for the flashy.”

Hobbs says they both “get a lot out of the emotional intensity of the work” and enjoy digging toward the heart of a character. That was particularly evident in an early finale scene in which Margaret organizes a birthday dinner for herself and invites her whole family to ask them for support in her ongoing marital war with Tony.

Rather than taking her side or offering sympathy, however, family members take turns gushing about Tony, causing Margaret to erupt and storm out.

“You introduced that beautiful shake in your hand, which showed just what you were talking about — that anxiety, that rage, that all came from you,” Hobbs says to Bonham Carter.

Luckily for both of them, Morgan asked Hobbs back to direct three more episodes in Season 4, including “one whole Margaret episode.”

“I was really lucky that I got Jess to direct,” Bonham Carter says. “She creates so much of the atmosphere, of your internal world, just through angles and edits and sound. It’s just like working with a great conductor, and I’m just maybe the first violin.”