If Watson had first met Moriarty:
Captain John Watson arrives in London after being discharged from the Army after being shot in the shoulder, leaving him with a psychosomatic limp requiring a cane and an intermittent tremor in his left hand. He walks into an Army friend, Colonel Sebastian Moran, and is invited out to drinks. While at the pub, he remarks that he is looking for a flatmate. Sebastian says that he already has one, but texts Moriarty to see if he knows someone. "James knows everyone and everything. He'll find you a flat, I'm sure." Moriarty replies that he does in fact know of someone who said the same thing just earlier that day, and tells them to meet him at Number Nine. Once they get to 9, John meets James and the three have tea. Moriarty explains that he and Sebastian were in business together as sort of odd-job consultants and middlemen. Moran was discharged from the Army and had met Moriarty who was then a rather bored mathematics professor. The two decided they were bored with conventional ways of making money, and gave a go at more unconventional means. As an example: James says he was hired by Mycroft Holmes to watch his younger brother, Sherlock, and create games to occupy the troubled puzzle-solver. Now, James is a middleman, so he wouldn't do the watching. Would John, perhaps, be interested? Sherlock is very interesting, in need of a flatmate, and it would mean James could take the job, of course John would then also be part of Unconventional Consulting. It wasn't anything major; make sure he doesn't do anything foolish, text James or Sebastian at UC if anything unusual happens, James will take care of the rest of the job. No worries. You get a flatmate, a small income, and almost no work. He can tell John's unsure about the breach of privacy, so he tells him he can go to the lab at St. Bart's morgue to meet him. He promises it will be fun. John goes. Sherlock asks for a phone. John gives him his. Sherlock finds a murderer. John shoots him. He goes out for drinks with Sebastian, he tells him nothing happened out of the usual. After drinks, when John is back at 221b Baker St. and Sebastian is at 9 Conduit, he tells James that John won't be of use. James disagrees. He wires money to the accounts of the children of the late Jeff Hope.
When Moriarty's involvement in certain events begins to be noticed and John realizes just what "games" James had in mind for Sherlock, John is torn and initially stays quiet. During the events of The Great Game, however, James texts John and asks him to reconsider the position with UC. John declines, calls him mad, declares his friendship with the two over, and refuses to answer any more texts. When he later leaves 221b, Seb is waiting to invite him to tea. "Oh, no, Doctor. James and I insist." Kidnapped at Number Nine, John is informed by Moriarty that he will be helping regardless of his refusal to volunteer. John asks how he could possibly have strapped people to bombs. "They all came to me, John. They asked for it. Every one of them asked me for their jacket, and every one got paid for it. I was providing a service." "But that old woman didn't have to die. She died, James." "Of course she died. That's what people DO!"
It is at this point that Sebastian holds up a vest and jacket. "Now. A while ago I provided a sort of matchmaker service to one Dr. John Watson. Our agreement was that you would be my eyes at 221b, but you haven't really said much over tea, Johnny, and the fee's gone up. Inflation, you know. I'm afraid eyes won't be quite enough. I'll need you to say. Just. What. I. Say. Seb? I think I could go for a swim."
Blahblahs where I fluff out canon with other detective series and smush it into BBC world with the vague idea of it complying to my wishes:
Watson and Moran were known to each other in the Army. Watson jokes on Moran's habit of cheating and is the only one to have noticed it. He kept quiet at the time because of greater loyalty to Moran than the 'not quite nice' fool who was played. Moran is constantly a small number of pounds in debt to Watson and they play cards as a way to pass time. They regularly have drinks out.
Moriarty and Moran live together in an expensive flat in 9 Conduit St., Mayfair, London. There is a rather alarming amount of competing floral and damask fabric for an expensive bachelor flat. Moran has hung a framed scenery photo of the Afghan desert over the mantle. It bothers Watson, but Sebastian looks at it with fond reminiscence. He also looks at James fondly and Watson assures him that "it's fine, Seb. It's all fine." The three have tea at Number Nine on occasion. Watson likes lots of milk or cream. Moriarty likes two sugars and lemon. Moran likes just a splash of milk. When there, John pretends nothing happens out of the ordinary with Sherlock, James pretends to be a gay maths professor, Seb pretends he isn't the second most dangerous man in London, and they all eat copious amounts of chocolate biscuits.
Moriarty's web page is not "The Art of Crime", but is instead based on Agatha Christie's Tommy and Tuppence newspaper advertisement: "Unorthodox Consulting: Willing to do anything, go anywhere... Pay must be good, no unreasonable offer refused."
(And because there is no canon Moran in BBC's series I am free to mold him how I want:)Watson describes Sebastian Moran as a sure handed shot who's favorite occupation is cleaning any weapon in his possession as "Idle hands are the devil's workshop, Watson." He speaks with a melodic tenor when pleased and a growl when cheated. He carries himself with the confidence of a private education and a military career and has the habit of often standing in doorways and making himself the Janus of any building, knowing anyone who enters or leaves as if it were his duty. He is taller than Watson and Moriarty, but a few centimeters shy of Sherlock, regardless, he looks up to no one but James. He has an angular face and strangely light caramel eyes, yellow-grey more than brown. With a constant five 'o' clock shadow, perennial tan, and sun bleached hair, he could pass for a classic Hollywood movie star. Like Samuel Spade in The Maltese Falcon, "He looks rather pleasantly like a blonde Satan."
I had Dean O'Gorman in mind b/c I think he'd make a pretty swell Sam Spade looking villain sharpshooter:
(Sorry, Dean. )