(A/N: Getting my feet wet in the Sherlock fandom with an alphabet set of prompts created by Written Sparks! Each letter has two prompts, either or both of which can be used; I am going to try to use both as often as I can. Since the chapters will be short, I will try to update frequently.)
A: Apprehensive /Alibi
The first time Sherlock says "John, I need an alibi," all John can think of is Sergeant Donovan's dire prediction that one day, it will be Sherlock leaving the bodies at the crime scenes. John doesn't tell Sherlock no, exactly, but he makes such a mess of things that it's months before Sherlock makes another unexplained request for an alibi.
By then John's confident that Sherlock's not going to go psycho killer on them, but he's got other worries. Worries about drug habits, and worries about giving an inch and having not so much as a mile but the length of the Atlantic Ocean taken. So he fumbles his way through another awkward rejection, and Sherlock seems to get the picture: that John, despite being the person he spends the most time with, is not the right person to ask to cover for him.
After two years of one-sided conversations next to the ultimate alibi - a gravestone - John hopes, prays, that Sherlock will ask him for an alibi again someday. He would give a different answer now. But Sherlock doesn't ask.