Unlike pretty much every other aspect of this novel, the narrator of The Idiot is pretty straight up and simple. It's the standard 19th century voice-of-reason style of drifting in and out of the heads of each character. It happens all the time in the text, so here is an example at random:
The prince was very nervous as he reached the outer door; but he did his best to encourage himself with the reflection that the worst thing that could happen to him would be that he would not be received, or, perhaps, received, then laughed at for coming. […]
Nastasia occupied a medium-sized, but distinctly tasteful, flat, beautifully furnished and arranged. At one period of these five years of Petersburg life, Totski had certainly not spared his expenditure upon her. He had calculated upon her eventual love, and tried to tempt her with a lavish outlay upon comforts and luxuries, […]. Nastasia did not reject all this, she even loved her comforts and luxuries, but, strangely enough, never became, in the least degree, dependent upon them, and always gave the impression that she could do just as well without them. (1.13.1-5)
Look how the narrator gets us first into Myshkin's state of mind—he is "nervous" and tries to "encourage" himself—and then just as easily dips into Nastasya and Totsky's mental back and forth—he is "calculating" that will come to love him, and she instead never grows "dependent."
We never get the sense that the narrator is holding anything back, nor do we ever feel like the narrator doesn't know something about the characters or the plot. It's all WYEIWYG with this narrative voice—what you expect is what you get.
What's the effect of this style? How would it be different if it were, say, in the first person? Is the fact that we get access to everyone's thoughts key to our understanding of the novel in any way?