Obviously, there is a lot of talking in the novel. Dostoevsky's characters are defined just as much by what they believe as how they present themselves through words. Each character has a distinctive way of speaking that comes through even in the English translation. Alyosha, for example, doesn't speak much, but when he does, it's short and direct, without any rhetorical flourishes. In contrast, Dmitri litters his conversation with poetic images and curses (usually involving the devil), while Ivan's speech is filled with his intellectual pretensions.
Not only do characters have distinctive speech patterns and mannerisms, they also have distinctive catchphrases – just as, say, the Homer Simpson has his "d'oh!" and Bart his "ay caramba!" Smerdyakov's catchphrase is "It is always worthwhile speaking to a clever man," while Ivan's is "All things are lawful." Even little Kolya Krasotkin has his own line against "sentimentality."
The characters not only define themselves through words, they interact and establish relationships with each other through them. Characters often echo each other – as when Alyosha echoes Ivan's words about "not accept[ing] God's World" or when Dmitri repeats Rakitin's explanation of how the mind works. Such echoing shows how words can actually construct the reality the characters live in, or perhaps even choose to live in. Dostoevsky often has different characters give competing versions of events. This is nowhere more clear than at Dmitri's trial, which is really a battle of words over what actually happened.
Dostoevsky's characters can seem at times to be more idea than human. Each character has a distinct point of view, and many of them are only too happy to talk on and on about their beliefs. Whole books and chapters are devoted to characters who open up about their way of looking at the world – Ivan's "Grand Inquisitor" fantasy, Zosima's life and teachings, even Dmitri's long-winded confessions to Alyosha. The novel foregrounds the characters' thoughts and opinions in order to expose how our beliefs can result in happiness or suffering. Perhaps, as Zosima says, we only have to believe that paradise is already here in order to achieve it on earth.
The narrator often steps in to give us key details about the characters. He not only places them within their social class – landowner or peasant, wealthy or poor – he also provides insight into the way their mind works. The narrator can also be quite obtrusive and direct – it's clear where his sympathies lie. For example, he will tell us that Alyosha is a "realist" (1.5.1) early on in the novel, so that when we hear the prosecutor tell us that Alyosha is a mystic, we are already skeptical. The prosecutor himself is depicted in an unflattering light: we are told, not shown, that he is "proud and irritable" and that he has a "somewhat higher opinion of himself than his real virtues warranted" (12.2.1).
The physical appearance of the characters often reveals their inner psychological state. Ivan's swaying gait indicates his ambivalence about faith and reason, while Dmitri's confident strides match his own brash and impulsive personality. Smerdyakov's "eunuch" physique differentiates him from the all-too-human passion of the Karamazovs.
The characters also express themselves through physical gestures, particularly when words fail them. Dmitri will pound his chest as a way to express his guilt and disgrace. This gesture is mysterious until we discover later on in the novel that he is actually touching the amulet in which the stolen money is hidden. Zosima's kneeling before Dmitri has a profound effect on both Dmitri and everyone else in the cell, as does Alyosha kissing his brother Ivan in a gesture of brotherly love and faith.