If someone is in the middle of shemoneh esrei and hears the chazzan saying “modim….” he should bow down [as per the rules previously learned] , but of course not respond in any way.
If the Kohanim start bentching birkas kohanim, he should stop and just listen.
Someone learning in a Bais Medrash may put his head down and nap , if that becomes necessary as an aid to his learning.It is preferable to distance oneself from the Aron HaKodesh
The following was culled from the Rav’s 60-second D’var Halacha after Ma’ariv:
The idea behind Eiruv Tavshilin is to begin, already on Erev Yom Tov, the preparations for Shabbos.
Generally, people use an egg (a cooked food) and a matzah (a baked food) as this would allow one to both cook and bake.
The way to do the Eiruv is to take the cooked and baked food, and to make the Brocha and to say the formula, found in the Siddur.
Lechatchila, the food you are preparing for Shabbos should be edible before Shabbos comes in, but B’dieved it does not need to be.
If you have guests who are BOTH staying and eating by you, they are automatically included in your Eiruv.
If you have guests who are ONLY eating by you, but are staying elsewhere, and they will be involved in food preparation, then the best thing to do (although b’di’eved it’s ok without this) is to be makneh part of the eiruv to them (having someone else–NOT a member of your family— “acquire (part of it) it for them,” not necessarily in their presence), and then to have them in mind with the Eiruv you are making.