When it comes to blessings that one is fulfilling by listening to the person who is reciting them (such as Kiddush and Havdalah), great care must be taken to give full ear to the blessing and not miss hearing even one word. All the more so, one must be careful to not talk in the middle, which would leave the person without Kiddush or Havdalah, or it would cause him to partake in some enjoyment without a blessing. Likewise, during the chazzan’s repetition, one must pay attention and properly listen to every single blessing, and to answer Amen as it should.
Those people who talk during the chazzan’s repetition commit several evils: (1) the mere talking in the Beis Hakeneses [synagogue] is forbidden; (2) It is a sin to talk in the midst of the chazzan’s repetition; (3) they lose for themselves the advantage of shomei’a k’oneh (“one who listens is equivalent to the one who recites”), and so they are like a fool who thoughtlessly throws away gold coins. The best thing to do is to hold a siddur (prayer book) in one’s hand and attentively listen to the blessings of the chazzan.