How is McCarthy able to make the postapocalyptic world of "The Road" seem so real and utterly terrifing?Which descriptive passages are especially...

The world seems so real because it is familiar to us.  The narrator and his son walk on a road, they encounter houses, towns, cars, semi-trucks, and stores with escalators and departments.  That is our world today.  We recognize it; there isn't anything there that we don't know.  Also, McCarthy chooses to have a narrator that has lived in both worlds; the world before the blasts, and the world after the blasts.  This helps to make the current scenes seem more real, because the narrator himself lived where we live, in a time like we live, and has seen the descent into chaos and despair.  Because he has been there, we believe it more.  He has dreams, memories, and a life from before, and is not trying to survive here, and that makes it real.  McCarthy also describes scenarios that are not too far-fetched, which keeps it feeling real.  The stores and houses are ransacked-a very likely and probably scenario.  People are so starving that they will do anything to survive-also believable.  The scientific side seems believable too-his descriptions of ash, cold, a distant sun, the petrified corpses, the fires, the brittle forests.  One descriptive passages says,

"Dark of the invisible moon. The nights only slightly less black.  By day the banished sun circles the earth like a grieving mother with a lamp."

All of that seems to fit a post-apocalyptic world, so it seems feasible and real.

McCarthy helps to make it so terrifying by hinting and alluding at the terrors before we actually see them.  Consider this moment, when he is remembering a conversation with his wife after the bombs where she says,

"Sooner or later they will catch us and they will kill us.  They will rape me.  They'll rape him...and eat us."

That passage alone strikes fear into anyone's heart, and McCarthy introduces it before they actually encounter any of these nasty "survivors," so that we too can feel the same dread when they run across the first ones of the book, in the truck.  Then, McCarthy makes good on his allusions by indicating that after he shot the man, his comrades boiled him and ate him:

"He found the bones and the skin piled together with rocks over them.  A pool of guts.  He pushed at the bones...they looked to have been boiled."

So, McCarthy makes the horrors true, as he describes it from the narrator's own eyes.  This is terrifying, especially as they keep running into these types.  We now know, and believe, what might happen, which makes their plight that much more grave.  I hope that those thoughts help; for your other questions, I suggest submitting them each separately, as the guidelines of this website allow for one a day.  Good luck!