Sam is suspicious... and rightly so. I think that Strider appreciates the suspicion, even if it means repeated defending of his intentions. Perhaps that is in part what leads Strider to confide the seriousness of Frodo's wound to Sam?
"I fear, Sam, that they believe your master has a deadly wound that will subdue him to their will."
Yeah... that fear is pretty spot on.
But more specifically, we're diving into the unnatural state of the Black Riders. Mortal weapons are of little use against the Riders, but the name of Elbereth strikes like a blade. The blade used to wound Frodo is found, only to melt away to just the hilt once Strider picks it up. Weathertop itself appears as an ambush point from the start, and now their relative peace merely the Riders confidence that there quarry is not long a free living agent. The wound that the Riders left is such that neither special herbs left from the days of the Men of the West nor elven magic are enough to stay it's dire effects.
Probably the best off of our party is the pony, who is much improved since his liberation from Bill Ferney and growing a bond with Samwise Gamgee.
Sam also further stands out as a disciple of Bilbo. He is quite curious about the world at large for a Hobbit, and has a quite fanciful and creative bent. The song at the site of the stone trolls is not insignificant.
The journey from Weathertop to Rivendell is still days away, days they must travel as swiftly as possible. The roads can't be trusted, but an elf-stone left on the Last Bridge seems to indicate at least safe passage over the River Hoarwell/Mitheithel. But the decision is made to avoid the road as much as possible until time to ford the Loudwater, the Bruinen of Rivendell. The land between the rivers are desolate and filled with ruins of great men who fell under the shadow of Angmar, long destroyed in the war that ended the North Kingdom.
Strider gives us a slight insight to his past in reflecting on Rivendell. "I dwelt there once, and still I return when I may. There my heart is; but it is not my fate to sit in peace, even in the fair house of Elrond." Not only that, but enough familiarity to say that "The heirs of Elendil do not forget all things past." The nice thing about a textual medium like a book is it easily allows for a slow build of reveal combined with greater exposition. We're not getting the full matter of Aragorn's heritage and birthright, but Tolkien is building up to that point.
Glorfindel is our first deliberate encounter with an elf, but spurred by that chance encounter back in the Shire. News traveled fast, with news of the hobbits wandering without a guide, Gandalf's absence, and the Nine abroad, reaching Rivendell far enough in advance to send out outriders nine days ago. Glorfindel is the source of the elf-stone on the bridge, left for the exact purpose that Aragorn took it for. I do wonder that even without the threat of the Black Riders, if the hobbits could have made it this far without a guide. The journey is far from straightforward. As it is, luck and fortunate allies has gotten Frodo and his allies this far.
Fortunately, elven steeds run swifter than the mounts of the Black Riders, and Glorfindel's steed is well trained. That training saves Frodo (and everyone, since his failure means the Ring is restored to Sauron), the siren call of the Wraiths pulling at Frodo nearly won, and his show of defiance would have meant little. But once crossed the river, one last show of defiance does seem to mean something.
And the river rises up in a "cavalry of waves" in which Frodo seems to see the forms of white riders upon white steeds in the froth. Whatever he saw, the Black Riders in the river are washed away, and those still on land soon follow as their crazed mounts leap into the flood. Frodo's faint into darkness in confusion could be from the toll the wound has taken on his mind and body, or maybe it's part of a last frantic pull on part of the Riders.
While looking at the film, for the time being we'll skip over the machinations at Isengard. Long story short, Saurman is up to no good.
Actually, the whole adaptation can be summarized as "long story short." An extended multi-day journey is compressed into one frantic day.
An injured Frodo has reappeared, injured by a dark blade who's evil magics work fast. Frodo is going cold and is "passing into the shadow, soon he'll be a wraith like them." No stated fear of the wound suborning Frodo's will, but a flat statement that the wound will ultimately transform our hobbit into a wraith. Out of all the hobbits, Samwise really is the one most likely to recognize the obscure "weed" Strider says they'll need.
No Glorfindel here, but instead Arwen playfully catching a frantic Aragorn off guard. It's a cute moment, but in terms of the bigger picture, it's a bit of an odd action on her part. She has spent the days searching for them, and knows that five of the riders are "behind" them on the road and the other four could be anywhere. I remember first watching the film and wondering at why Arwen glowed so radiantly when running to Frodo, as it turns out I completely forgot how Glorfindel appeared to Frodo's eyes.
Actually watching this I'm shocked at how young Liv Tyler looks, smooth cheeked with a softness of residual baby-fat. Orlando is likely to also appear shockingly young once he appears again. The elves in The Hobbit are played by older actors, ones that retain something of an ageless quality but that no longer have the softness of recent youth.
I know quite a few people who are emphatically not a fan of Arwen's expanded role in these films. I have a bit of reading and re-watching to go, but this substitution I'm not bothered by. Within it we are clued in to a long history between Aragorn and Arwen, one where body language hints at one time or perhaps hoped for intimacy. Jackson is giving us a piece of Aragorn's story arc that needs some sort of presence for later events in the story.
The Frodo here couldn't stay on any elven steed, no matter how well trained, by himself. So Arwen rides ahead with a barely conscious hobbit in front of her. Jackson has made the mounts of the Black Riders a bit more evenly matched to the swift elven steed, though that likely is more for drama than anything else. The ambush itself seems largely missing, but there is a mix of open plains riding and chaos of sparse forests before they reach the river. There is no question about the river responding to a mystic call nor the shapes of horses within the angry froth. The river comes in Arwen's response, washing away the riders, and we end with Frodo collapsing as Arwen holds him tight.