Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third-Person Omniscient
This third-person narrator constantly weaves in and out of different character's minds, giving you a god's eye view. Even when the narrator doesn't give you all the pertinent details about a character, it isn't because the narrator doesn't have this knowledge, but because withholding knowledge amps up dramatic effect.
The narrator even uses a movie-like technique in the way it'll often pivot between different characters. For example, after the narrator lives inside the mind of Ossipon for much of Chapter Four, it switches into the Professor's mind at the beginning of Chapter Five, and shortly after latches on to Inspector Heat. We recommend some Dramamine for those of you who get motion sick: the point of view in The Secret Agent moves around like the Alice In Wonderland Tea Cup ride.
Conrad's mind-hopping technique is something you find all over late-Victorian novels, such as in George Eliot's Middlemarch. But Conrad wasn't just following a Victorian literary fad; this narrative voice helps us to empathize with some pretty unsympathetic characters. When we're in their heads, we kind of have to feel what they're feeling, and that makes them human.
There's one weird moment early in the book when the narrator steps right in and talks in the first person: "But of that last [point] I am not sure, not having carried my investigations so far into the depths […]" (2.2). This moment doesn't fit in at all with the rest of the story, and it seems to show that Conrad is worried about what people might think if he keeps showing off his knowledge of pornography shops. It's as if he's saying, "Here's exactly what the inside of a porno store is like…uh… or so I've heard."