Before The Witcher ever became a Netflix series, its star was a massive fan of the books. Henry Cavill is such a big fan that he's done little things here and there to make sure that the series stays true to its source material; including sneaking in some dialog from the books that wasn't originally going to be included in the show.
While it's very clear that The Witcher show is based more on the books than the video game, the series still can't include everything that comes from its source material. However, throughout the project's first two seasons, Cavill has worked hard to make sure that other fans of the books see the story they expect to see. He's talked about places he wants the series to go and apparently has made subtle tweaks to what's going on in the show, sometimes without even asking permission.
One such story of the little changes the actor has made came from a recent interview with a Polish language publication that has since been translated by Redanian Intelligence. In that interview, he said that he snuck some dialog in that wasn't in the script and he did so without telling anyone else involved with the show. It turns out that despite the speech being a surprise, it must have gone over well as it was left in the show.
Henry Cavill - who is described by his castmates as a "Witcher bible"- said there was a specific set of words that captured him with their poetry. He explained that in the books, there's a scene where Geralt loses consciousness and in a vision or a dream, meets the embodiment of death. "Then, beautiful words are uttered about crossing the meadow and the fog," Cavill said.
The actor went onto say that he included the words in Season 2 of The Witcher as something Geralt says mainly so he could get the words into the show. He added that after he delivered the line on camera, he expected to have someone yell at him for the adlib, but no one seemed to react or know that wasn't the original intended line in the script.
It's possible that because Cavill is viewed as such an expert on The Witcher and its source material, that he gets a bit more leeway than another actor would get. Because of his fandom, those around him might realize that when he adds something like this, it's going to fit just fine into the tone and tenor of the show.
The Witcher season 2 is available on Netflix.
Source: Redanian Intelligence