Symbolism, Imagery, Allegory
From the first moment that Reynie meets him, Sticky falls back on his anxious habit of polishing his spectacles so often that it eventually becomes a symbol of his nervousness. So as soon as we see him reaching for his cloth, we know without a doubt that Sticky's got a serious case of nerves going on.
The cool thing, though, is that after he discovers there is bravery in him after all and confronts the Whisperer (and Mr. C), Sticky never reaches for his cloth again—so then it's the absence of this gesture that becomes symbolic. It means that Sticky, who may still get nervous or scared, has on some level overcome his anxiety.