Rosemary Sutcliff‘s The Eagle of the Ninth historical novel tells a story which is not followed to the letter by The Eagle film-makers Jeremy Brock (writer), Duncan Kenworthy (producer) and Kevin MacDonald (director ). In fact, there are several differences, although I believe the film captures well both the essence of the story that Rosemary told and the essence of the novel. Since Rosemary was a writer not a film story-teller , I believe she would have respected decisions by film-makers to change aspects of story.
A lover of good stories (and amongst other things, film Westerns) she might have understood that MacDonald and colleagues sacrificed absolute accuracy to her book to the requirements of a good story on film. She would have respected them choosing a good film-story (as they saw it) over absolute faithfulness to the book. My evidence? This from her about the historical accuracy of her books:
Since I am a writer, not an historian, I will sacrifice historical accuracy. I really very seldom have to do it, and then it is only a matter of perhaps reversing the order of two events, or something like that. But if it does come to the crunch, I will choose a good story over absolute historical accuracy.
Source: Interview with Rosemary Sutcliff by Raymond H Thompson (and also here on this blog)