Dickens starts building suspense in "The Signal-Man" by using imagery. repetition and punctuation. An example of Dickens distinctive imagery is the description of the signal-man as being distorted: "his figure was foreshortened and shadowed." "Foreshortened” is a term in art that means to make certain parts shorter than in real life to create the correct allusion of distance and angle to give the correct perspective. The narrator leaves the reader with a very vivid image of the signal-man looking somehow wrong compared to how he should look--he is somehow distorted. This builds suspense because it prompts questions in the reader's mind about how and why and in what way.
Dickens also repeats significant words to build a certain sensation and, with the sensation, a feeling of suspense. For instance, in the first twelve paragraphs, "down is repeated twelve times: "down the Line"; "down in the deep"; "down and speak"; "down at him"; "down to him"; etc. He also uses punctuation to build suspense by creating a cadence that inspires a feeling of tension and suspense, like in the following passage where punctuation and the repetition of "and" create a feeling of reluctance and hesitance in the first-person narrator and in the reader:
I resumed my downward way, and stepping out upon the level of the railroad, and drawing nearer to him, saw that he was a dark sallow man, with a dark beard and rather heavy eyebrows.
Doyle and Dickens both start their stories in medias res, in the middle, so we are without knowledge of circumstances or characters at the out set of the story. He uses distinctive imagery, as Dickens does, along with another technique. At the start of "How It Happened," Doyle builds a compelling image through the narrator's description of his experience--a description inexplicably coming to us through "She": "She was a writing medium. This is what she wrote." This opening both builds suspense for all the unexplained things it suggests (e.g., who is she, what is a writing medium) and foreshadows what will come thus adding to the suspense. The narrator, framed by the "writing medium," imparts a distinctive image when he says that some parts of his memory are "most distinct" and some vague, "like some vague, broken dreams," making it hard to tell his whole story: "That is what makes it so difficult to tell a connected story."
Doyle also uses another technique when he starts focusing upon physical events--rather than upon impressions and psychological experiences--that happen as they occur in chronological order and through the experience of the first-person narrator. For example, Doyle puts emphasis upon the brakes giving out; being unable to throw the car into reverse; Perkins placing his hands upon the wheel; the wheels of the car being on the bank; and other such physical details. The result is a build up of suspense as one physical event after another impacts the narrator and Perkins--and the reader at the same time. The narrator starts with "the big motor, with it's glaring headlights and glitter of polished brass, [ominously] waiting for me outside," then moves to "I clapped on both brakes, and one after the the other they gave way," and ends with "I whirled round my wheel with all the strength of my wrists. ... my right front wheel struck full on the right-hand pillar," with many, many more suspenseful physical events in between.