In my women in history class, we read "The Yellow Wallpaper" by Charlotte Perkins Gilman, and talked about what we thought was happening during it-what was real, what happened to the narrator, and what it all meant. Then, we watched a film adaptation of the short story, but I didn't like it nearly as much.
"The Yellow Wallpaper," like Secret Sharer, does not provide a clear narration of what is literally happening. Discussions and personal thought allow the reader to think of multiple ways the text can be read, and to probably pick a favorite. Arguments about what is being symbolized are very cool, because many sides and opinions can find textual evidence.
For both of these stories, having to defend my opinion and look through the text for other people's really helped me get into the story in a Nabokovian way. I was trying to visualize what was being unclearly narrated, and it was pretty cool.
Then, we started watching the film version of "The Yellow Wallpaper." To film a story means having to make decisions about what is visually going on. There can be some level of mystery, but it is by no means as ambiguous as written words. I was not engaged with the film. It didn't seem able to make the same comments as the text, and it was not nearly as exciting. I think that if I had not read the story beforehand, I would not understand half of what was happening.
If there was a film version of Secret Sharer, then I think I would feel the same way. The filmmakers would have had to decide if Leggatt was a real, physical being, or a ghost, and how Leggatt and the Captain interacted. The Captain's internal troubles would not be able to come through clearly.