If you think Kit Harington’s role in the film became scaled back during script developments or rewrites, that’s not the case. During an interview with The Direct, “Eternals,” screenwriters Ryan and Kaz Firpo revealed that they always planned to include Dane in the film because of his relationship with Sersi in the comics, but he was never meant to have much screen time in the movie. The screenwriters also noted in the interview that the character has more of a minimal role for this outing because the Eternals dealing with their past serves as the plot’s big focus, and it didn’t need much of Dane for that.
“I think that the movie in the present day is a lot of dealing with the demons of the past for the Eternals family,” Ryan Firpo told The Direct. “So it was always the intention to keep them alone together because that’s the best way for them to be forced to deal with all of their issues from the past. So there was never an iteration where Dane went on the journey with them.”
While it does make sense why Harington’s role is minimal, there’s the belief that his character will play a much more significant factor for the MCU in the future. By the end of the film, it appears that Dane will eventually suit up as his alter-ego in the comics, the Black Knight. Furthermore, Harington has expressed hopes that his role in “Eternals” will be the beginning of something more in the MCU.