In Vayikra 9:22 the posuk tells us that Aharon lifted his hands and bentched Klal Yisroel. The Gemara in Sotah 38a says that we learn from the word “Ko“ that the bracha of the Kohanim is given while the Kohen stands. The Gemara says we learn this from a hekesh from the brachos that were given on Har Grizim where the Torah says “Yamdu.” Tosfos in Sotah 38a brings in the name of the Ri that if a Kohen gave the bracha while sitting, the bracha is ineffective as we compare it to the word “leshareis,” which means to serve, and a servant serves while he is standing. We find in Zevachim 23b that if a Kohen did the avoda while sitting, the Avoda is mechulleles. Therefore, a Kohen who duchened while sitting would need to redo it and make the bracha again.

The Shvus Yaakov says the above hekesh is not Min HaTorah, it is Mi’deRabbonon and the hekesh is just an asmachta. The Rambam is therefore lenient when it comes to an elderly Kohen or a sick Kohen permitting them to sit, bdieved. The Be’er Heiteiv, in the name of the Kneses Hagdola, Panim Meiros and Noda BiYehuda argue with this position. They all hold that standing is an ikuv and if a Kohen can’t stand, it is better for him not to duchen at all.  

The Magen Avraham in Orach Chaim 128:22 says that only the Kohanim need to stand, but the tzibbur may sit. The Mishna Berura 128:51 brings the Meiri and the Eshkol that the tzibbur must also stand. The Rokeach explains the above posuk by saying that Aharon lifted his hands “El Ha’am,” the tzibbur moved from their designated place to stand before the Kohanim. The Geonim write that it is required for the tzibbur to take a step closer to the Kohanim to receive their bracha.

Rava, the son of Rav Huna, says in Sota 31a that once the Sefer Torah is opened one is not allowed to talk, even in Halacha, as the posuk tells us when they opened everyone stood. Standing means being quiet, as we find in Iyov 32:16, “I waited and did not speak because we stood at attention.” The Shailos U’teshuvos Binyamin Zev 163 quotes this Gemara and says that despite the fact that we learn from the posuk that one has to be quiet, we still learn the pashut pshat that one needs to stand. The Rema 146:4 says that one should be stringent and stand. The Mishna Berura 19 says that despite mei’ikar dina one does not need to stand; it is nevertheless preferable to stand. A person should feel as if he was mekabel the Torah on that day. It will help the person concentrate. In the event that standing will cause a lack of concentration, it is better for the person to sit. The Arizal would sit during the entire Krias HaTorah including while the Oleh said Barchu

The Shulchan Aruch 422:7 paskens that one should stand while saying Hallel over the year. The Bais Yosef explains that we learn this from a posuk in Tehillim that says “Hallelu” and then says the words ”Ha’omdim baleilos.” The Pri Chadash says we learn this from the fact that saying Hallel is like a hagadas eidus, and eidim usually testify while standing. The Taz s.4 brings a different proof as the Yidden said Hallel on Erev Pesach in the azarah and no one besides a king from Shevet Yehuda was allowed to sit in the azara. The Toras Chaim in Hilchos Brachos 219:4 writes that according to the Rambam one may sit during Hallel as the Rambam does not mention standing for Hallel.

              We find in the Poskim that the Hallel we say during the seder on Pesach is said while seated. One reason for this is that Hallel is said in a split way: part of it during Magid, and the rest at the end of the Seder. The reason we split Hallel is because the first part of Hallel discusses Yetzias Mitzrayim, which is apropos for Magid. On the other hand, the rest of Hallel discusses the future geula, which is apropos for the end of the Seder where we daven to be redeemed for our final redemption. Another reason is that since the Seder is done when we are leaning back “be’heseiva,” the Chachamim were not matriach a person to have to stand during Hallel of the Seder.

May we be zocheh to say Hallel while we are makriv the Korban Pesach in thanks ofr the final redemption.