As mentioned earlier on, the words of a wizard are to be feared indeed. Saruman's voice is heard as a delight, as wise and reasonable, its listeners compelled to agree. All other voices sounding harsh and uncouth, voiced disagreement kindling anger. He is the epitome of the enemy seeming fair while being foul. He speaks to the insecurities and fears, offers alliance and salvation.
Not all are swayed, but Saruman does his best to discredit the voices of reason. Theoden nearly falls to Saruman's thrall, but perhaps his recent experiences in breaking free helped him find his way out again. Their king's resistance breaks the spell of Saruman's voice and composure, turning nasty and disparaging. Reminds me of a relationship I used to be in.
Actually, joking quips aside, this chapter resonates with me very strongly. I've been within the orbit of/under the influence of people who speak and behave like Saruman. Not in content, but in methodology and intent.
Again, as in earlier chapters, mercy and redemption is a theme. Gandalf gives Saruman a choice, asks him to come leave Orthanc, free to go with leaving the key to the tower and his staff as bond. To offer that to a Wizard so twisted that he refers to the ents as wood-demons and who was in deliberate collaboration with Sauron is mind-boggling.
In this case, unlike many of the others, we get an immediate positive effect regarding the show of mercy. Gandalf proved that Saruman's power and influence has waned, and with the upper hand, he would have benefited from either Saruman's refusal or his acquiescence. In this case he gains respect for his mercy and awe for his power over his enemy.
I do wonder at the full powers of the ents that they can guarantee no escape for Saruman. I do recall a certain wizard escaping off the top of Orthnac, and Saruman does have flying allies of his own.
This chapter is done quite succinctly and with little fanfare in the film. Among other things, there is a significant lack of audience beyond our core Fellowship. So instead of the great battle of wills, Gandalf and company ride up to Treebeard, confirm Saruman's captivity and his continued guardianship by the ents, acknowledge Saruman's lack of power, and pick up a Palantir.
It is perhaps worthwhile noting that this is our first actual encounter with Saruman, and our first chance to compare Christopher Lee's visage to the description in the text. I would probably approve even if they didn't match up, since it's Christopher Lee. Fortunately, it's a good match.
The Palantir might as well be a perfect match for the text, a dark crystal orb with a fire within. Pippin's intense interest to it can be seen on his face.