I've only seen bits and pieces of the films, and what I've seen seems mostly true to the books. The one character who's nothing like how I imagined zem in the books is Bellatrix Lestrange.¹

The thing that stands out to me about Bellatrix in the books is zir extreme pride, with the power to back it up. Ze is almost always in control of every situation – the only time ze isn't is when ze's around Voldemort or Snape, who are even better at controlling situations. Ze lectures Harry on how to use the Cruciatus Curse in the middle of a battle. Ze declares zir loyalty to Voldemort while being sentenced to life in Azkaban. Ze's second only to Voldemort in the number of named characters ze kills by sheer magical skill. Ze never passes up an opportunity to impose zir unforgiving, blood-purist ideology on other people.

In the films, ze's nothing like that; instead, ze's an “out of control woman” stereotype. The actor who plays zem describes zem as “really naughty” and “incredibly infantile”. Ze just does whatever ze feels like, and ze always feels like causing havoc and destruction. There are a few elements of that in the books, but the films add a lot more that wasn't in the books at all, and reduce the other aspects of zir character.

There's an interesting question here about the concept of agency. The actor describes Bellatrix-in-the-films as “liberated”, but to me, it feels like ze lacks agency. Unlike in the books, ze's never steering the course of the narrative – instead, ze's going along with the narrative created by the other characters.

In my process of writing Voldemort's Children, the important thing about each character is not the personality quirks ze happens to have, but the ideology ze attempts to impose on the world.