The first question might be: why would I want to hide my protagonist’s thoughts from the reader? This usually happens when the protagonist knows something that would spoil the climax of the book if it were divulged too soon.
Let’s say your protagonist is a police detective (Joe) who’s the lead investigator in the department’s attempts to discover and stop a serial killer. If you’re writing from the detective’s point of view, let’s say, third-person limited, then the reader knows only what Joe knows, sees, experiences, thinks about, or learns through conversations with other characters.
However, the author of this story has a surprising climax in store for readers when it’s divulged in the last chapter of the novel: the detective is, in fact, the killer, and one aspect of Joe’s warped motive is the “fun” of misleading fellow police officers (Bob, Sam, and Bill) without appearing to do so.
So you see the problems here?
First, how do we account for the Joe’s time when he’s killing somebody and getting rid of the evidence. One way to try and do that is to tell the story through multiple points of view, say–one per chapter. We have a Joe chapter, followed by a Bob chapter, followed by a Sam chapter, etc. If, none of the killings takes place during a “Joe chapter,” does that solve our problem of hiding what Joe is doing?
No, because when we do come to a Joe chapter–whether it depicts Joe and others searching a crime scene and/or Joe talking about the evidence and the suspects–it’s unrealistic (I say impossible) for Joe to do any of these things without thinking about the fact he committed the crimes and, perhaps, even wondering whether he hid the evidence or the bodies well enough.
The minute he does the natural thing and thinks about any of that, the big surprising ending has been spoiled. If he never thinks about it (and doesn’t have a split personality), the readers are going to feel cheated when they finally learn Joe’s the killer.
I’ve been thinking about this lately because one of the main characters in my novel in progress has some secrets I don’t want the reader to know until late in the book. My solution is to avoid writing from that character’s point of view. Will it work? I don’t yet know. Suffice it to say, it was obvious to me from the beginning that I couldn’t let the reader know directly what this character was thinking.
Maybe you can think of other ways of hiding the main character’s thoughts from the reader. My solution might crash and burn. It’s hard to know how these kinds of things will turn out.