This is one area where I think they can improve on the book.

In the book, I think most readers immediately assume that the mysterious lady in green they meet on the road is the bad guy. And then, once we meet the crazy prince, it's pretty obvious it's Rilian. I suspect this was not intended by Lewis.

So, if the movie can figure out way to preserve the mystery longer, I'd be open to that. I would like Rilian calling on the name of Aslan to be more of a reveal.

Not that all is lost in the book. The real drama comes from seeing whether or not the characters will follow the signs. That's still there even if the mysteries are not very well hidden.

At the moment, I can think of two ways that they might deal with this problem:

1. The audience knows that Rilian was lured away by a woman in green, but the Narnians don't. This would probably require cutting out Drinian and starting the movie with Rilian's backstory, showing him meeting the LotGK by the fountain and probably ending when his horse returns riderless to Cair Paravel. Later in the film, the owls tell the questers about the Queen's death by a serpent's sting and Rilian's subsequent disappearance after going hunting for the snake, but there is no mention of the mysterious woman because nobody knows that detail except the viewer.

I think this would mostly side-step the facepalm-inducing feeling when Jill, Eustace and Puddleglum don't suspect the LotGK and the faceless knight when meeting them on the ancient road. And even then, if the questers not wondering about the identity of the knight is still enough to cause frustration, you could have the LotGK accompanied by a small company of knights instead — one of them being Rilian, and the others human-sized gnomes.

2. Change the dynamic of the scene on the giant road. Instead of having the LotGK and the black knight approach together, have the LotGK approach them alone at first. Her mood and manner should be very like how it appears in the book, but when the black knight appears in the distance, an ominous figure watching them through the mountain mist, her demeanor suddenly changes and she stiffens. She says that she must go and vanishes into the mist in the direction of the knight. It would appear to the audience that this woman is in the power of the menacing-looking knight, that she is good and he is evil, when in reality she's the one in control and she leaves because she doesn't want Rilian getting close to Narnians just yet.

I really like both of your ideas, Rose! I especially like the first one because it requires so little change from the book. All that is needed is a prologue (likely to happen in any event) and the removal of Drinnian's story. We still have a parliament of owls, but the scene is shortened, something that would probably need to happen anyways.

I think someone might have mentioned this somewhere, but what if the audience knows nothing more than that the queen was killed and that Rillian disappeared while hunting the serpent? Due to the signs, the audience will at once suspect that the handsome young man in underworld it the prince, so at that point, during his fit, he could have flash-backs of the fountain and of the witch luring him away. In the book he actually remembers a beautiful pool of water, and I imagined that maybe it was near the fountain where he was seduced. That could be the cause of his torment, to have his mistake play over and over in his mind.

I think someone might have mentioned this somewhere, but what if the audience knows nothing more than that the queen was killed and that Rillian disappeared while hunting the serpent? Due to the signs, the audience will at once suspect that the handsome young man in underworld it the prince, so at that point, during his fit, he could have flash-backs of the fountain and of the witch luring him away. In the book he actually remembers a beautiful pool of water, and I imagined that maybe it was near the fountain where he was seduced. That could be the cause of his torment, to have his mistake play over and over in his mind.

That's an interesting thought. I'm trying to imagine what it might be like for the viewer when meeting the LotGK and the knight on the giant road, having no idea that a woman figured into Rilian's disappearance at all. The audience would probably be more inclined to think that she was a good guy, or at least that sending them to Harfang had been a mistake. It could make her reveal as a snake even more shocking. In the book, Rilian gives a brief recap of his story after he slays the serpent, so it's possible that a flashback to when he met the LotGK might be included then.

I like the idea of starting the movie with scenes in Narnia showing the story of Rilian's disappearance, similar to how Prince Caspian began in Narnia, but it would also be interesting for it to begin in England and the audience is just as blind as Eustace and Jill regarding the quest. Discovering Narnia alongside them might have more impact.