Dr. Johnson's peculiar mannerisms were always a subject of comment, and they often seemed beyond his control. A series of spasms or grimaces would cross his face (tics, we would call them now), as well as compulsive gestures (as shown in Joshua Reynolds early portrait of Johnson above), and a constant rolling of the legs even when standing still. He obsessively had to touch every post he passed in the street, and went back if he missed one. At one point in his life he saved all his orange peel. I think all these symptoms, coupled with his social difficulties and hard to control temper, might lead now-a-days to a diagnosis of Aspberger's Syndrome.
This demeanor of his (and his slovenly clothes) led the artist William Hogarth, when he saw him at the writer Samuel Richardson's house, to suppose Johnson an "ideot" who had been put in Richardson's care. But when Johnson stalked forward to join in Hogarth and Richardson's discussion and took up the subject with eloquence and learning, Hogarth was astounded and really thought the "ideot" had been "inspired", that is, taken over by a spirit!
Johnson also had the habit of checking the bookshelves in whatever house he visited, sometimes to the extent of becoming oblivious to his hosts. He once told Boswell that when he was at any gathering and the talk turned to society, he would "abstract" himself and think of "Jack the Giant-Killer", one of the children's chap-books from his youth.
Johnson was an amateur physician and "chymist", at a time when most scientific knowledge could be attained by one man. He had a laboratory set up at his lodgings, and Boswell would sometimes be greeted by the chemical stinks of one of his experiments. When Johnson had a stroke, he quickly made some Latin verses to test whether his mind had been impaired (one of his greatest fears). He found that they were poor, but that he knew they were poor; he concluded his critical faculties were undamaged.
Johnson may well have been at least partially a model when Arthur Conan Doyle was creating Sherlock Holmes. Certainly Watson ends his tribute to his friend after his supposed death at Reichenbach Falls by calling him "the best and wisest man I have ever known." This is exactly how Boswell concludes his famous biography of Johnson.
And now to finish my own work with the words Johnson used to Boswell in a fit of pique when his friend had been unusually persistent in his questions: "Sir, you have only two subjects of conversation, yourself and me; and I am heartily sick of both!"