One of the things I loved best about Psycho was how carefully and slowly Bloch revealed key points. How we not only received little hints, all the way through, about Mrs. Bates and her role, but those hints built on each other. Each new hint or mention revealed just a smidge more information than the hint before it. And at no point did he simply re-hash information we already had.
The first hint about Mrs. Bates and Joe Considine comes on page 117. It's the first time Considine is mentioned, and Norman is thinking about appearance. He then reflects, "Oh no it wouldn't! Because Uncle Joe was dead...Funny how it had slipped his mind."
Considine's name doesn't come up again until page 161, when we find out just a touch more about his death, this time that Considine and Mrs. Bates together committed suicide. Interestingly, this revelation doesn't come from Norman, but from the Sheriff. At this point, we're pretty sure Norman is an unreliable narrator, so it's kind of an, "Oh, okay," moment for the reader. Here we have a reliable (or at least more reliable than Normal) character giving us information in a factual matter. We can believe it. We think we've caught up.
The next hint builds directly on this, on page 172, because it instantly reputes the Sheriff. The information we assumed was reliable is suddenly unreliable when we go back to Norman's POV, when Bloch writes, "Norman had fooled the Sheriff the first time, when everything had been much harder. This time it should be even easier, if he remembered to be calm." And on the next page, "How he'd fooled the the first time! And yes, he fooled them just as easily again...."
The intersting thing about that hint was that we don't yet know what the truth was. We only know that what we thought was true (the Sheriff's POV) is no longer true.
It's not until page 190-191, that Normal tells Sam that they drank the poison together and that he was, "...in the hospital a long time. Almost too long to do any good when I got out. But I managed."
At this point, the information once again matches up with the Sheriff's account, but Norman is so unreliable a narrator by now that we know there's something else going on, and the sentence of "almost too long to do any good" gives us a great hint of what actually happened.
And then finally, on page 208, we realize Mrs. Bates is a corpse and he killed both Considine and Mrs. Bates.
I think Bloch's style here, the way built on previous hints, and especially the way he refuted information from a reliable narrator with an unreliable narrator, really lent to the suspense of the book. Especially the psychological suspense of figuring out what was happening with Norman's mother.