Say your signature writing move is hysteria. Just full-on, insane in the membrane, screaming matches between characters who in an ideal world would have iron-clad restraining orders out against each other. What do you do to really make those scenes land? Well, you do it by constantly keeping everyone in a state of unrelenting tension, followed naturally by moments of complete breakdown.
Does this make for the best way of getting characters to seem like real people? No, of course not—but Dostoevsky isn't going for high realism here. Instead, he seems to be doing his darndest to figure out where the utmost limit of human emotion lies. Let's look at a scene to see how this works:
The prince suddenly approached Radomsky.
"Evgeny Pavlovitch," he said, with strange excitement and seizing the latter's hand in his own, "be assured that I esteem you as a generous and honourable man, in spite of everything. Be assured of that."
Evgeny Pavlovitch fell back a step in astonishment. For one moment it was all he could do to restrain himself from bursting out laughing;
[Myshkin] smiled strangely; but suddenly and excitedly he began again: "Don't remind me of what I have done or said. Don't! I am very much ashamed of myself, I—" […]
"What's the matter with him? Do his fits begin like that?" said Mrs. Epanchin, in a high state of alarm, addressing Kolya.
"No, no, Lizabetha Prokofievna, take no notice of me. I am not going to have a fit. I will go away directly […] I see that I am out of place in society—society is better without me. […] it is impossible for people not to laugh at me sometimes; don't you agree?" […]
All present stood rooted to the earth with amazement at this unexpected and apparently uncalled-for outbreak; but the poor prince's painful and rambling speech gave rise to a strange episode.
"Why do you say all this here?" cried Aglaya, suddenly. "Why do you talk like this to them?" She appeared to be in the last stages of wrath and irritation; her eyes flashed. The prince stood dumbly and blindly before her, and suddenly grew pale.
"There is not one of them all who is worthy of these words of yours," continued Aglaya. "Not one of them is worth your little finger, not one of them has heart or head to compare with yours! You are more honest than all, and better, nobler, kinder, wiser than all. There are some here who are unworthy to bend and pick up the handkerchief you have just dropped. Why do you humiliate yourself like this, and place yourself lower than these people? Why do you debase yourself before them? Why have you no pride?"
"My God! Who would ever have believed this?" cried Mrs. Epanchin, wringing her hands. (3.2.1-17)
This passage is pretty textbook. Just look at the way no one can have an emotion without it being as big and broad as emotion could be. The prince isn't just agitated, he is "sudden," and "strangely excited"; instead of taking Radomsky's hand, he "seizes" it. The same goes for all the other characters, too: Mrs. Epanchin isn't just concerned, she is "in a high state of alarm"; Aglaya isn't just mad, but instead she is "in the last stages of wrath and irritation." Everything is amplified and broadcast at top volume to keep the characters constantly at boiling point and the reader's heart rate up.