According to Imam "Al-Shafi'i", it's based on Sunnah to say "Wa-alikum Al-Salam" if someone says it during the "Khotobah". Also it's based on Sunnah to say "Yarhamukum Allah" if someone sneezes, and I'm sure the same goes for saying "Ameen" and of course offering blessings to our dear prophet (pbuh) when saying "Sallalahu alihi wasallam". All the above is from Sunnah and Imam "Al-Shafi'i" says that there is no "Ithim" and isn't considered "Lagu".
The only thing that is prohibited while the Imam is offering his "Khotobah", is to talk to your fellow Muslim, or to tell him "hey listen" or to make any other conversation that doesn't relate or mentioned in the Sunnah. (Wa-Allahu A'lam)
Source: Fiqh Al-Sunnah