In a 1954 letter to Allen Ginsberg, Burroughs wrote "The idea of shooting a glass off her head had never entered my mind consciously, until - out of the blue, as far as I can recall - (I was very drunk, of course), I said: "It's about time for our William Tell act. Put a glass on your head, Joan."
So there is reason to think that there was no "routine". Burroughs came up with the idea there and then. It's also worth noting that Burroughs changed his story a number of times over the years.
People might be interested in reading James Grauerholz' "The Death of Joan Vollmer Burroughs: What Really Happened?" (https://www.yumpu.com/en/document/read/6810449/the-death-of-...). Grauerholz was was friends with Burroughs and his text ends up trying to exonerate Burroughs - however, it is fairly evenhanded and also mentions a good deal of information that paints Burroughs in a negative light.
Personally, I lean towards the theory that Burroughs did shoot Vollmer accidentally. However, I think that evidence pertaining to the incident is much more ambiguous than how it is usually presented in articles that deal mainly with Burroughs himself as their theme.
For what it’s worth I think killing Joan was an accident. By most accounts he loved her dearly.
just like Mary down in Mexico on All Souls Day."
I am unsure why Natalie Merchant changed her name from Joan to Mary.
Ministry - Just One Fix https://www.youtube.com/watch?v=XYYGKCanqfA
Ministry - Quick Fix https://www.youtube.com/watch?v=NYKiD03kVGw
https://www.youtube.com/watch?v=F9NDxYf122I
It might not grab you at first, but if you spend some time with it, there's a whole weird world that opens up in there. And Burroughs is a masterly reader.
He also contributed one vocal track and a bunch of lyrics to the excellent Tom Waits album "The Black Rider".
[0] - https://classic.esquire.com/article/1971/9/1/life-with-fathe...