Mescal, who is right-handed, had to learn how to play the guitar backwards for the role, as Beatles legend McCartney is left-handed.
Speaking to Rolling Stone in a recent interview, Gracie said Paul's guitar skills have been "so amazing to witness".
"He's like an alien," she continued. "I mean, he's better at guitar backwards than I have been at guitar my whole life."
Gracie and Paul have kept much of their relationship private since they were first romantically linked in 2014.
Elsewhere in the interview, the Hit the Wall singer revealed she has recently warmed to the idea of having children one day.
She explained that as she has grown older, she has come to understand how families can work through challenges, "reap the benefits of fighting to love each other and see each other", and how the relationships that follow can feel "very earned" and "very lucky".
However, for now, Gracie is focused on making music.
"I would do it every day," she shared. "I would choose it every day. I crave those days in the studio so much. I think after this album cycle wraps up, the way that me, today, imagines planning my life will look way more like that."
Once she's finished promoting the album and her upcoming tour, the singer hopes for "long stretches in one place".
"I see long, creative projects - I feel desperate to collaborate with people and be around people that are not shy in their expression," she stated.
All four Beatles biopics, focusing on each member of the band, will be released in cinemas in April 2028
Paul Mescal and Barry Keoghan confirmed to play Paul McCartney and Ringo Starr in Sam Mendes’s Fab Four biopics.
At the end of Sony’s CinemaCon presentation on Monday night in the US (April 1 in the UK), director Sam Mendes took the stage to make a pair of blockbuster reveals for his four upcoming Beatles films.
After a year of speculation, Mendes has confirmed that the Fab Four will be portrayed by Harris Dickinson, Paul Mescal, Barry Keoghan and Joseph Quinn. Dickinson will play John Lennon, Mescal will play Paul McCartney, Keoghan will play Ringo Starr and Quinn will play George Harrison.
Saoirse Ronan and Aimee Lou Wood, Mia McKenna Bruce, Anna Sawai as the wives of the Fab Four.
Saoirse Ronan, Mia McKenna Bruce, Anna Sawai, and Aimee Lou Wood have been cast in Sir Sam Mendes' Beatles biopics as the wives of the Fab Four.
Lady Bird star Saoirse will play photographer Linda Eastman, later McCartney, while How to Have Sex actress Mia has been cast as Maureen Starkey nee Cox.
Shogun star Anna will take on the role of Yoko Ono, while the White Lotus actress Amy will play Pattie Boyd.
Mendes has said: “Maureen, Linda, Yoko and Pattie are four fascinating and unique figures in their own right – and I’m thrilled that we’ve managed to persuade four of the most talented women working in film today to join this amazing adventure”
Sam Mendes is making four upcoming Beatles films – each focusing on one member of the Fab Four – and earlier this year, it was revealed that the legendary band will be portrayed by Harris Dickinson, Paul Mescal, Barry Keoghan and Joseph Quinn.
