First off, you can have that dialog occur in a confined space which, for logistical as well as social reasons, is difficult to get out of. Dostoevsky uses the cell of Father Zossima, Alyosha's elder at the monastery. You can have your well-meaning young protagonist bring his embarrassing family to meet the person he reveres most in the world; and you can have that family include the drunken buffoon of a father who immediately freaks out in a verbal paroxysm of shame before this representative of God. You also have the snide family friend who provokes Fyodor further; the Impressive Atheist brother (Ivan) who has published an article on the separation of church and state--he's against it, as it turns out; and the mysteriously absent brother, Dmitri, on whose behalf this meeting was actually called in the first place. Oh, and throw in a passel of women who've come to seek Zossima's blessing, and a girl in a wheelchair who has mischievous eyes and a thing for Alyosha. Also, make Zossima ill, practically on death's doorstep, so that Alyosha's worried he's going to collapse at any second.
All of these people articulate some theological position or conundrum; in the women's and Fyodor's cases, their dilemmas cause them the most excruciating suffering. For instance, one woman is in agony because the last of her children has died, and she cannot stop mourning for him; worse, she feels it's wrong to keep mourning the way she does, because she knows the baby's in heaven with God, and yet that doesn't console her. Zossima gives her some rather good advice, all in all:
Consolation is not what you need. Weep and be not consoled, but weep. Only every time that you weep be sure to remember that your little son is one of the angels of God, that he looks down from there at you and sees you, and rejoices at your tears, and points at them to the Lord God; and a long while yet will you keep that great mother's grief. But it will turn in the end into quiet joy, and your bitter tears will be only tears of tender sorrow that purifies the heart and delivers it from sin. And I shall pray for the peace of your child's soul.
The whole episode, however, is anything but peaceful; as soon as he's calmed this poor soul (and I believe we are supposed to wonder for how long), there's another one begging for his help. And that's before he returns to his cell and starts a lengthy discussion with Ivan on his article, while Fyodor threatens to melt down again at any minute, and Dmitri, who loathes Fyodor, could still show up.
In short, the monastery is not a place of calm reflection. At least not on this day. It's a place of chaos, where people desperate to love and obey God clamor for answers, even as others stroll in to call religion's very purpose into question. And it's this background of chaos that will, I think, keep us reading through Ivan's and Zossima's arcane debate. They're not having this discussion on some mountaintop. The consequences of even the finer points of their conversation are all around them. The discussion format is still necessary, not only to lay out all the ideas for later examination in the narrative, but to begin to reveal Ivan's character. But Dostoevsky has earned our patience with and interest in these large blocks of dense text, with the tragicomedy that has preceded them.