One of the most elusive characters in all of the Lord of the Rings film adaptations is the Mouth of Sauron. Not only does he have one of the shortest appearances on screen, but also, very little is explained in the movies about his character and what purpose he serves for the dark lord, other than treating with the enemies at the gate to try to get them to fall prey to his master's demands. He is well known for his over-sized, oozing lips, cracking at the seams from the poisonous words that spill forth out of them.
But there are many questions that arise from the scene in which the Mouth of Sauron appears. There are questions of Aragorn’s honor with the very uncharacteristic beheading, questions of exactly what terms Sauron might have laid out had the Mouth had but a chance to relay them. Finally, there's the question as to why Gandalf, who is known to be highly loving and compassionate towards his dearest friends, would silence his companions when they grieve for the loss of Frodo, their kin.
The scene comes about when the Mouth of Sauron holds up the hobbit-sized chain-mail, made of the rare and precious Mithril that ensures it is unmistakably Frodo’s, a gift handed down from Bilbo and his own adventures with the dragon in Erebor. The Mouth states, “I have a token I was bidden to show thee,” and holds up the Mithril jacket, which was last seen in the hands of an orc, escaping the ruckus in the tower of Barad-dur after searching Frodo, whom they had captured thanks to Shelob’s nasty sting. When he reveals the mail, Pippin gasps “Frodo!” and Merry cries “No!”, to which Gandalf commands “Silence!” This seems very out of character for Gandalf. Many viewers thought that he would be more moved by the prospect of Frodo’s demise, and what it might mean for the quest to destroy the ring of power, and the fate of Middle Earth as a whole.
But after silencing the hobbits, it is clear that Gandalf is affected, deeply, by the revelation. The Mouth continues to goad them, describing the great pain that Frodo suffered. “Who would have known one so small could endure so much pain?” The shot zooms in on Gandalf’s face, who looks heartbroken, with tears in his eyes. Why then would he have silenced the others' grief so hastily, considering how terribly he was feeling his own?
The answer is quite simple: he had to protect the quest above all else. At that moment, all hope must have seemed lost, and it would have only made sense that the enemy had already discovered the ring in Frodo’s possession. However, the Mouth made no reference to the evil object. Gandalf, therefore, had to err on the side of caution and make sure that neither of the hobbits revealed anything in their shock and horror at Frodo’s fate.
After all, why would Sauron have sent The Mouth to parlay with them, if he already had possession of the ring, the one thing that would give him ultimate control and power? Gandalf’s silencing of the two, usually far more comedic and less tragic hobbits, is not out of disrespect or unfeelingness towards their sorrow, but out of the hope that somehow the ring journeys onward, despite the Mithril seeming like evidence to the contrary. Perhaps he hoped that Sam had continued on with the quest, even after Frodo had been caught, or perhaps he hoped beyond hope that Frodo had escaped their clutches alive, just as Gollum had done several years before. Either way, the wizard had to stick to his Istari duty and put the fate of the world before his and his companion's own grief.
There is also possibly some part of Gandalf that can still feel a tether to Frodo, and knows that what The Mouth is uttering are lies. The wizard later tells the others "Frodo is alive, I can feel it," which suggests that he has some inexplicable link to his young hobbit friends. He is known as Gandalf the wise, and perhaps this is the moment that he becomes wise to the enemy's use of fear tactics and baiting to get them to divulge their plan, in order for the true master to be able to foil them at the last moment. And it is indeed wise that he does silence them. As the audience knows, Frodo and Sam are actually safe and together, that Sam had the ring whilst Frodo was captured, and that they are still on their way, at that very moment, into the fires of Mount Doom, with Gollum hot on their heels.