So, I just watched the 1962 tv movie of Ninety-Three (which you can download for four euros here).
SPOILERS for both book and movie:
It's generally pretty faithful to the spirit with some weird changes in details. It entirely cuts the Paris scenes/Book II, so no Marat-Danton-Robespierre scene in the cafe. Cimourdain and Gauvain's backstory is instead quite effectively conveyed by a few comments from Gauvain and by the general tenderness of their relationship.
The famous "loose cannon" scene is changed to an accidental fire instead, I assume for budget reasons, but the subsequent confrontation between Halmalo and Lantenac is kept intact, though transferred to dry land.
The music is very good- I notice French films set during the revolution use the Marseillaise less (though there's a lot of similar martial music) and Ca Ira more than American and British films.
The actress who plays Michelle Flechard does a fantastic job and really shows her heroism. The film also ships Michelle Flechard/Radoub (as I do) and ends with Radoub saying he'll explain the republic to her and them laughing together as the children play. The roles of women are improved by having the vivandiere survive the massacre and actually get to fight (!) at Dol (she's present at the final battle as well, though I didn't see her in action there, whereas at Dol she gets a gun and is fighting with the men. I loved that!). The Dol scene generally was well done in terms of cinematography. There are some dramatic falling-off-things deaths by extras, which the extras clearly had fun with.
The ending is where the biggest and imo worst changes happen- Lantenac's big speech/rant defending his royalism is cut, and Cimourdain and Gauvain's dialogue the night before the execution (which is only the thematic center of the book) is significantly cut and altered to basically (as far as I could make out the French without subtitles) to:
Gauvain: What if the law of the universe is against us?
Cimourdain: God is in the revolution.
I'm sure I missed some stuff though due to not really speaking French. There was also an interesting addition- Lantenac has a box of poison which he plays with and then throws away in prison. Gauvain plays with it and throws it away when he in turn is in prison, and Cimourdain uses it to commit suicide at the end.
Despite the changes to the end I really enjoyed the film overall (and I liked the Michelle Flechard/Radoub stuff too). I think the three main actors did a good job portraying Lantenac, Gauvain, and Cimourdain.