I asked this very question recently to a group of book editors. They disagreed with what was written here. For context, here was my original post:
So, I have a question and it’s a situation I use a lot, but find contradictory information about.
“I hate this refrain,” John said, as he went and sat in the corner of the room by himself.
The question is the rule for the comma before the word “as.” I think “as” should be seen as a conjunction here. Any specific rules on this one.
Here is what four professional book editors said on this topic:
I feel, and I must confess I'm not sure about the rule here, that if the actions are simultaneous - his speaking and moving to the corner of the room - there is no need for a comma. 'As' can be tricky if actions do not happen at the same time, however. (E)
The word "as" is merely an adverb that is being used to move this sentence forward. It's not setting off a descriptive expression or clause like "that is" or "namely" or "however" would, which all require some punctuation. In this case, no comma is required and creates an unnecessary pause there. (E)
Here, ‘as’ is being used as an adverb to mean ‘at the same time’. No comma. When it’s used to mean ‘since’ or ‘like’, then it takes a comma. (E)
A fourth editor chimed in to clarify:
As Editor 1 and Editor 2 explain it in their comments earlier in this thread. There is no one rule for commas after dialogue tags. It depends on the words used and the grammatical construction of the sentence. If the sentence were ''“I hate this refrain,” John said, sitting in the corner of the room by himself', then the comma would be necessary. (E)
As you can see, the general consensus is that in this context the word “as” would not need a comma. No editors argued for the comma.