In chapter 5 of Avi's The Man Who Was Poe, Edgar Allan Poe, calling himself Auguste Dupin, has Edmund take them to a café. Prior to going to the café, Dupin and Edmund had just been in the rented room across from Edmund's building where Edmund found a button from his sister's shoe, serving as proof she had been forced into the room. As Edmund sits in the café contemplating Dupin's recently shared conclusion that his sister had been kidnapped, he sees Dupin "staring across the room." The man Dupin is staring at is the night watchman, Mr. Throck.
As Edmund continues talking to Dupin, asking Dupin if he thinks his aunt's murderer and sister's kidnapper are the same person, Dupin keeps his eyes locked on Throck. Dupin continues discussing the facts of the case with Edmund, explaining that there are two men involved and that the ones who delivered the message to his mother in England and the ones who kidnapped his sister are the same. Just as Edmund asks, "Do you know who they are?," Dupin again glances at Throck while saying, "I have a good idea."
Since Dupin keeps staring and glancing at Throck, the reader is lead to believe Dupin knows Throck to be somehow involved in the mystery. Hence, just by looking at Dupin, who is staring at Throck, the reader is able to deduce that Throck is somehow a suspicious character.