May one go up for an aliyah in the middle of Kriyas Shema?

If a person is called up for an aliyah while he is in the middle of Shema, its brochos, or bein haprakim, the Rishonim disagree about whether he should go up. Shulchon Aruch is stringent, but Mishnah Berurah quotes Achronim who say that the custom follows the other opinion unless one is in the middle of the pasuk of Shema or Boruch Shem. If possible, a person who is called should stop at the end of the perek or minimally complete the idea that he is in the middle of and then go up for the aliyah. He should not read along with the ba’al korei. According to some Achronim, if he receives the Maftir aliyah he may also read the Haftorah if that is the custom.  

סעיף ד' ס"ק כד-כו ביאורים ומוספים דרשו 17

May a Kohein go up to duchen during Shema?

L’chatchilah, a person who is in the middle of Shema or its brochos should not be called up for an aliyah. If he is the only Kohein or Levi in the shul, however, the poskim disagree about whether he should be called up. Some say that he should, since if he is not called up his yichus will be questioned. The halacha is that he may be called up if he is bein haprakim. It is best if he leaves the shul to avoid the issue before the gabbai has a chance to call him up. Regarding Birchas Kohanim, a Kohein should only go up to duchen if he is the only Kohein there. Otherwise, he should leave the shul. Some poskim allow him to duchen even if there is another Kohein there. 

סעיף ד, ס"ק כו, ושעה"צ ס"ק מד; ביאורים ומוספם דרשו, 18

Can one interrupt in the middle of Vayomer during Ma’ariv?

The prakim of Shema and its brochos are as follows: the first brocha, the second brocha, the first parsha of Shema and the second parsha of Shema. Upon completing the third parsha of Shema, one must immediately say the word emes. It is forbidden to answer any greeting or even take a break between Elokeichem and emes. Some Achronim say that the word v’yatziv must be said immediately after emes. From that point until Shemoneh Esrei, one is considered to be in the middle of a perek. At Ma’ariv there is an additional perek-- the brocha of Hashkiveinu. According to the basic halacha, the third parsha does not have to be said at night. As such, some Achronim propose that it has the status of bein haperakim even when one is in the middle of saying that parsha. The rules of not interrupting in the middle of the prakim also apply when Shema is said independently of its brochos. If one repeats Shema with the brochos after he has already been yotzei, the poskim disagree about whether the rules of prakim apply.

סעיף ה, ס"ק כז ו־כט, וביה"ל ד"ה ואלו; ביאורים ומוספים דרשו 37

• During the prakim of Shema and its brochos, a person may only answer Amen to the following brochos: Hakel Hakadosh, Shomeya Tefilah, the brochos of an oleh l’Torah (according to some), and the brochos of Birchas Kohanim.

• During the prakim of Shema and its brochos, one should answer Amen Yehei Shmei Rabba and Amen at the end of half-Kaddish. One should also respond to Kedusha and Modim D’rabonon. When responding to Kedusha, one should only say the pesukim of Kodosh and Boruch.

• According to the Shulchon Aruch, a person who was unable to put on tallis and tefillin before davening may put them on with a brocha if he is bein haprakim. Once he has already started the last parsha of Shema, however, he should put them on without a brocha. Rema allows tefillin to be put on with a brocha in the middle of the parshios of Shema or the brocha of V’yatziv. 

• Should the word emes be repeated?

• May one put on his tallis right before Shemoneh Esrei?

• Should one answer Amen to Go’al Yisroel?