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In the movie, which comes from director Ridley Scott, Damon stars as Jean de Carrouges, a French knight who was involved in the last judicial duel in the country's history, while Affleck plays nobleman Count Pierre d'Alençon.
Speaking to Entertainment Tonight, the pair revealed there was originally one scene in the film that saw Damon's character landing a kiss on Affleck's, but that this was revised slightly to show him kneeling instead.
Affleck said: "In the original actual version of that scene - the way that ceremony actually took place was that you kissed everybody on the mouth.
"And we had that in the script."
Damon then said: "That would've been our first on-screen kiss."
Affleck lamented: "It's going to have to wait."
They didn't even get to film the moment, as the moment never made it into the final script after Scott decided to change things up - ultimately opting to have Damon kneel before Affleck.
Affleck explained: "Ridley thought it would be distracting, and his instincts are pretty good."
Damon continued: "Yeah the two characters really hate each other and that was really fun.
"In fact, the scene where I kneel before him, Ben just improvised this thing where I started to say my line and he just interrupted me and goes, 'Closer.'
"And I have to kind of get up and kneel again in front of him.
"And that was just - he just did that and just kind of blurted that out, and it wasn't in the script."
The actors joked that the kneeling is something that 'goes on at home too', with Damon quipping: "I actually have to kneel before him.
"Once he started directing, that's actually how I have to get in his house."
The new film also features Jodie Comer, who plays de Carrouges' wife Marguerite.
After Marguerite claims she has been raped by Jean's former friend, Jacques Le Gris (Adam Driver), de Carrouges calls for trial by combat, and the fate of the three is decided in the last legally sanctioned duel in France.
Comer said getting the opportunity to work with Alien director Scott was a 'dream come true'.
Speaking at the London premiere of The Last Duel, she said: "It was incredible. It was such a dream come true to see him work behind the scenes and see the mastermind and how he makes his decisions. It was really fascinating.
"And also he has four or five cameras rolling the entire time, so as an actor you are incredibly spoiled because, especially if you are doing material that is emotional or a bit darker, then you are not spending a lengthy amount of time doing it. It was quite an efficient way of working."
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