Alistair Moore is an old friend of Sherlock Holmes. Born in 1946, he was a struggling actor and worked at Talisman Books. He died from a heroin overdose in 2014 and is survived by his partner Ian, and son Jeremy.
When Holmes was ten years old, he wrote a letter to Alistair who then starred in a radio drama, to tell him that he was the only one in the cast that truly captured the Yorkshire accent. They later met, so Alistair could coach him in the nuances of the Yorkshire accent. They then became friends. When Alistair moved to New York City in 2006, they mostly kept in touch by e-mail. Holmes did drugs, but Alistair hoped he would grow out of it. Years later, Holmes showed up at Alistair's apartment at night, so high he could barely speak. Alistair got rid of his drugs and looked after him. Holmes could barely stay conscious and muttered Irene Adler's name again and again.
When Holmes' sober companion Joan Watson won't believe that Sherlock's father Morland never keeps appointments, he hires Alistair to act as his father, to play a trick on Watson. Morland doesn't show up for dinner so the plan works. When Alistair asks if Joan is sleeping with Sherlock, she knows something is wrong, and realizes Alistair isn't Morland. Joan later shows up at the bookstore Alistair works at and asks him about Sherlock's past. Alistair tells Joan that he and Sherlock are friends, but Joan comments that Sherlock doesn't have any friends. Alistair then tells her that Sherlock doesn't have friends in the traditional sense, but that he drops in and out, and appears at odd moments to make outrageous and highly specific requests. He also tells her that she can't expect Sherlock to relate to you the way others might. ("Flight Risk")
You've been spending quite a bit of time with him haven't you?
Alistair unexpectedly dies of a drug overdose after being sober for over thirty years. His death causes Holmes to question his own sobriety and whether it can be maintained. Holmes imagines Alistair at several junctures during a case. However, he comes to realize that Alistair's choices were his own and not his responsibility. At Alistair's grave, Holmes mourns and imagines Alistair apologizing for the manner in which he died. ("No Lack of Void")
So who are we watching for?
He knows nothing let him sleep on.
After Alfredo Llamosa goes missing, Holmes questions whether he knew Alfredo's mood since the last time he saw Alistair before he overdosed, he thought he was in good spirits. ("A Controlled Descent")