In "The Signal-Man," one of the narrator's earliest observations of the signalman relates to his strange behaviour and agitated manner. We see evidence of this early in the story, as the narrator makes his way to the signal box:

His attitude was one of such expectation and watchfulness that I stopped a moment, wondering at it.

Similarly, the narrator notices this a second time, as he observes the signalman carrying out his duties. He is unable to conjure an explanation for such behaviour, though he clearly finds it compelling:

He directed a most curious look towards the red light near the tunnel’s mouth, and looked all about it, as if something were missing from it, and then looked it me.

Finally, though the narrator does not yet know the reason for the signalman's strange behaviour, he begins to wonder if he is mentally ill:

I have speculated since, whether there may have been infection in his mind.

It is also worth noting that these quotes are effective at building suspense as the narrative develops. This is because they foreshadow the final appearance of the ghost in which the signalman tragically loses his life.