How does Arthur Miller show isolation in The Crucible? Besides the land being far from anywhere, is it the characters that are isolated from one another?

The characters in the play are indeed isolated from each other. The witch craze has taken such a viselike grip on Salem that everyone's forced to look out for themselves. What had been a close-knit community has been turned by the prevailing hysteria into a collection of atomized individuals, each one desperately trying to avoid false accusations of witchcraft.
This is what often happens in modern totalitarian societies. If people turn against each other, then that makes them less likely to challenge the existing authority. It's much easier, and safer, to make false accusations of witchcraft to settle an old score than it is to stand up and say that the whole witch hunt is nothing more than a tissue of lies. And even when John Proctor does precisely that, he remains, if anything, an even more isolated figure than ever before, as no one's prepared to join him in his act of selfless martyrdom.

John Proctor has certainly become more isolated. He tells Abigail that he's hardly left his farm in the last seven months, likely as a result of his affair with her being found out by his wife. It seems as though he's been trying to prove to her that the affair and the feelings that went with it are over. We also learn that Proctor does not care for the Reverend Parris; he makes it quite clear in Act One while at Parris's house to observe the ailing Betty. However, in Act Two, Reverend Hale comes to the Proctor home to ask some questions, and he reproaches Proctor with his habitual failure to attend church: he's been to services only twenty-six times in seventeen months. Moreover, his third child has not been baptized. Proctor's severe dislike of Parris has kept him isolated. It is also, likely, his embarrassment about his affair with Abigail that has kept him distant; even his wife says that Abigail cannot pass Proctor in church without him blushing.