a. John was talking incessantly and Bob was drinking shot after shot of vodka making me feel very uncomfortable.
b. John was talking incessantly and Bob was drinking shot after shot of vodka, making me feel very uncomfortable.
The two sentences are the same except for the comma after the word vodka.
Can we tell if John's talking and Bob's drinking are both making me feel uncomfortable or only Bob's drinking?
Does the comma change anything?
You need a comma there to indicate the consequence (both of John's incessant talking and of Bob's heavy drinking):
b. John was talking incessantly and Bob was drinking shot after shot of vodka , making me feel very uncomfortable.
A comma could only be done without if the V-ing described the way in which one of the actions was performed.
However, it is often required even in those cases, especially when the main verb is already in a progressive tense or when there is an adverb of manner in between.
The clue, then, is that the absence of the comma will not enable the consequence meaning that is required in the sentence at issue.
