If you are learning in Shul, and the tzibur starts saying aleinu, you stop and say it together with them. Aleinu should be said with awe, concentration, and proper kovod.
There are still people who see fit to joke around precisely when the sefer Torah is being brought out to the bimah to layn in. Besides the issur of קלות ראש in a Shul, it is also decidely a total lack of kovod for the sefer Torah! Maybe this is their first opportunity to be permitted technically to talk since the davening started—nevertheless, it remains ossur בהחלט!