Q. My author wants to know whether a comma is called for in constructions like the following, where a conjunction follows the dialogue tag but doesn’t introduce an independent clause: “It’s very clear,” she replied[,] and moved off to a nearby tree. I tend to think it’s needed but can’t articulate why. I also think it needs to be “she replied, and THEN moved off.” Can you help?
A. This is a common question. Normally, a comma wouldn’t be required before the conjunction in a sentence that features a compound predicate (see CMOS 6.23):
She replied and moved off to a nearby tree.
But in dialogue, a speaker tag is usually set off from a quotation by a comma; it makes sense that, by a similar logic, the speaker tag would also be set off from any action or other narration that occurs in the same sentence:
“It’s very clear,” she replied, and moved off to a nearby tree.
And that’s what we’d advise—unless your author favors a style that’s notably light on commas and asks that you leave commas like that one out. In either case, you could add then after and, as you suggest.
Or you could switch to the present participle for the action verb, in which case a comma would be required:
“It’s very clear,” she replied, moving off to a nearby tree.
If the sequence of events is important, add a word like before or while:
“It’s very clear,” she replied, before moving off to a nearby tree.
Whatever approach you use, aim for consistency across like contexts.