‘Amen’ should be said immediately after the Chazzan has concluded the last word of the berachah. This is to ensure that full concentration is paid to the berachah to which one is saying amen, i.e., that the amen not be a rote, automatic, answering.
One must make sure not to answer amen too hastily,before the Chazzan has completed the final syllable. This is ossur, forbidden.
We read in this week’s sedra about the mitzvah of Tefillin. Tefillin is one of the holiest objects that a Jew is likely to own. It is forbidden to act casually while wearing them, which includes sichah betayla, idle talk.
Think–you are in a Shul, you are wearing Tefillin–are you really feeling so disconnected that you can just stand there and shmooze?