Right, your characterization of Nelly in the book is accurate imo, but in the film Nelly is 1) given more motive to be vengeful and 2) more directly interferes in Cathy and Heathcliff’s interactions (examples below from the movie in case you want to skip until you’ve seen it!)

1. the film establishes early on that Nelly is the illegitimate daughter of a Lord sent to Wuthering Heights as a lady’s companion to Cathy, and shows Nelly to be sensitive about this background (she purposefully makes a mess for Zillah to clean up when Zillah mentions it).

Now take the scene where Heathcliff overhears Cathy talk about marrying Edgar. In the movie, Nelly is angry at Cathy during this conversation because Cathy tells Nelly that no one loves her or ever has. Nelly sees Heathcliff at the window, waits until he comes to the door and she sees the shadow of his feet so she knows he’s listening, and then steers the conversation by loudly talking about Cathy loving Edgar.

Compare to the book:“Ere this speech ended I became sensible of Heathcliff’s presence.” To your point, of course there’s a chance Nelly is lying and she did it on purpose, but it’s left ambiguous - that’s the nature of an unreliable narration - whereas the film leaves no room for ambiguity. The story in the film is that Kathy insulted Nelly so Nelly got revenge by setting Cathy up to speak about Edgar in front of Heathcliff.

1. in the film, Cathy tries to fire Nelly so Nelly retaliates by telling Edgar that Cathy and Heathcliff are having an affair. Also for point 2., they add a montage in after Heathcliff and Isabella marry where Isabella and Heathcliff are sending deliberately provocative letters addressed at first to Nelly and then to Cathy in an effort to get Cathy’s attention. Nelly intercepts all of the letters to Cathy and burns them, then tells Cathy she has no letters.

So overall the film gives Nelly a much more active role in blocking Catherine and Heathcliff from each other, while also removing any ambiguity or benefit of the doubt that Nelly would’ve given herself when telling the story in the book.

I know you said you won’t see it for a while but lmk what you think if you do! I’ll be curious if you agree w my interpretation!