To summarize: it's not interrupting* to speak up in agreement and support of another speaker, but it's definitely interrupting to stop their train of thought and disagree.
*Depending on the cultural norms, of course. The author notes that in NYC it's OK, but I wouldn't try it a lot of other places.
*Depending on the cultural norms, of course. The author notes that in NYC it's OK, but I wouldn't try it a lot of other places.