Russell, 54, is currently filming The Loudest Voice in the Room, a limited series in which he plays the now deceased Fox news chief Roger Ailes, whose career ended after he was accused of sexual harassment.
Both Russell and Sienna, who plays his wife Elizabeth, have been photographed on set wearing facial and body prosthetics that give them both dramatically different appearances from usual.
But their looks didn’t seem to alarm Sienna’s daughter, Marlowe, who visited the set and was seen happily interacting with Russell in between takes, even making off with his prop walking stick at one point.
Russell and Sienna have become so close he gifted the 37-year-old a beanie from his beloved Rabbitohs’ footy team, and the insider notes, “she’s never not wearing it”.
However, there’s one person who’s less happy about the budding relationship.
New Idea has learned that Terri is upset about the pair's increasing closeness even though she’s no longer romantically linked with Russell.
“Terri can’t help but be jealous. She and Russ still talk often over the phone and he has been talking more and more about Sienna,” reveals the source.
Sienna and Marlowe’s father, Tom Sturridge, were together for four years before ending their relationship in September 2015.
Since then she’s been linked to Brad Pitt, director Bennett Miller and New York book editor Lucas Zwirner.
She has admitted to having regrets about her past relationships and behaviour, alluding to her 2008 romance with married father-of- four Balthazar Getty.
In 2014 she admitted how she’d been lucky that Hollywood had forgiven her, saying, “I’m very lucky to have a second chance in that town ... It was really bad. What was going on in my private life was not an easy thing to read about. People don’t want to see films with people they don’t approve of in them.”
Meanwhile, Russell – who once called Terri “one of the most fabulous women on the planet” – has always been tight-lipped about his dating history since splitting from Danielle Spencer in 2012.
Russell says the main reason for his reluctance to be open about his relationships is to protect his children, Charles, 15, and Tennyson, 12.
“Like, OK, I have broken up with their mum, but I don’t want to parade a whole bunch of girlfriends in front of them,” he said in 2017.
While Terri and Russell have both admitted publicly to loving each other as good friends, they have always denied having romantic feelings.
Yet if Russell does take things further with Sienna, the insider insists Terri will be heartbroken.
“Deep down, Terri hoped that they would one day reconcile and find their way back to one another.
"Seeing how close he and Sienna have become is a really big obstacle to that now."
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