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Colin Firth and Stanley Tucci are in love. No, really, ask them yourself. The chief virtue of Supernova (in theaters Jan. 29, on digital Feb. 16) — a tender British movie centered on longtime couple Sam (Firth) and Tusker (Tucci), and their struggles with Tusker's early-onset dementia — is the chemistry between its two decorated leads. Warm and singular and complicated, Sam and Tusker's relationship is just as it would be for any two people who've loved each other so deeply for so long, and just as it is for Firth and Tucci. "Our love for each other, our respect for each other, that intimacy was already there," explains Tucci, 60. "When you have a really good friend, it's like a lover. You know a lot about each other, things that other people don't know — things that even spouses don't know — and that makes you incredibly close."

Written and directed by Harry Macqueen (2015's Hinterland), Supernova follows Sam and Tusker on a final road trip as they reconnect and meet up with family and friends. An established novelist, Tusker seems accepting of his fate even as his memory lapses make it impossible to put word to page. Sam, on the other hand, can't accept how quickly the end of their journey is approaching. His stubbornness and looming grief fuel some of the film's most heartbreaking clashes, in which reality pushes back against fantasy; they're wrenchingly and subtly acted by two men at the top of their game.

"[Tusker is] a man for whom his sense of control is very, very important to him," says Firth, 60. "My encounters with dementia, and they are very close to home for me, made me recognize that. I found that very truthful and authentic." Adds Tucci, who heavily researched the role: "When you see people with it, they do what the actor should do…play against it and laugh about it, even, before falling apart. Then the person who's taking care of them ends up having the reaction you think the person who has dementia would have. That's the thing that makes it so complicated and so painful."

It helped to have a great friend to lean on. Firth and Tucci met 20 years ago when they played Nazi officials in the 2001 HBO movie Conspiracy; they now live close to one another in London. The decision to collaborate on Supernova came after Macqueen sent the script to Tucci, who loved it and passed it on to his old pal. And how do you say no to the great Stanley Tucci? "Well, you don't," quips the Oscar-winning Firth, last seen in The Secret Garden. "This was wrapped up in Stanley and a two-decade-long friendship that we've had."

While hardly so easy on their own work in the movie, Tucci and Firth can't say enough about each other's performances. "I think Stan has surpassed himself here, which is quite a bar to surpass," says Firth. (Demurs Tucci: "That's enough of that. I'm getting uncomfortable.") And while there's a "craft" to it, as Firth notes, Supernova's beauty lies in the realistic, tough portrait of true love brought so effortlessly to the screen. "In a way, most of our job was done for us simply by being as close as we are," Tucci says. "It was a gift."

Illustration by Hsiao-Ron Cheng for EW.

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