The most commonplace face is most difficult to identify. How is this illustrated in "The Red-Headed League"?
I suppose this could be related to the identity of Spaulding and his real motive for working in the pawnshop that belongs to Wilson. What is so fascinating about this case is the way in which the job that Wilson had was actually all a complete ruse. The Read-Headed League is a complete fiction that was too good to be true, as Holmes recognises. As a result, this leads him to inquire as to the precise details of the case and how in particular Wilson found out about the job. The key role of Spaulding, Wilson's assistant in the pawnshop, made it absolutely clearly that he was behind this job as a distraction in order to get up to something in Wilson's absence.
Holmes then is able to go forward from here to work out the rest of the details of this crime. He goes and looks at the knees of Spaulding whilst also establishing that a tunnel has been build by knocking his stick against the floor. What is initially shown to be a baffling mystery is something that features a "commonplace face" of greed and robbery. Holmes, through applying his ruthless logic, manages to uncover the mystery and the rather mundane motive for the creation of the Red-Headed League.