I have been very happy to have comments by the art historian Marianne Gilchrist - aka "Silverwhistle" - on several of my posts. In September she published an article on the website ARBR (Amis de Robespierre pour le Bicentenaire de la Révolution) which pulls together some of her thoughts on that all-important question: what did Robespierre look like?
According to Marianne, Robespierre's most easily recognisable feature is his nose, which was narrow and pointed, and slightly turned-up at the end. This can be seen clearly in the better authenticated portraits, for instance the famous Carnavalet portrait, the bust by Deseine and several of the higher-quality engravings:
A particularly precious piece of evidence is provided by the little physionotrace engraving of Robespierre (reproduced by Buffenoir, "Portraits de Robespierre", Annales Révolutionnaires (Paris 1908) vol.1(4) p.646-7. Plate 31A)