Shulchan Arukh Orach Chayim · Siman 146

English

To Not Speak at the Time of the Torah Reading. Containing 4 S'ifim: It is forbidden to exit [the synagogue] and leave the Torah while it is open. But between one person's [aliyah] and another, it is permitted.

English

Once the reader began reading from the Torah scroll, it is forbidden to speak even words of Torah, and even between one person [i.e. aliyah] and the another, and even if one already finished the parasha. And there are those who permit to "grind out" (meaning: to learn) quietly. And there are those who say that if there are ten who are listening (meaning: they are paying attention) to the Torah scroll, then it is permitted to discuss (words of Torah) (Beit Yosef in the name of Mahari"a). And there are those who permit it for those for whom Torah is their profession. And there are some who permit it for one who turns one's face away before they open the Torah scroll, who makes self appear that one does not want to listen to the Torah scroll, but rather [wants] to read [i.e. learn], and begins to read [i.e. learn]. And to read "shnayim mikrah, v'echad targum" [i.e., the practice of "reading each verse twice, and the Aramaic translation once"] during the Torah reading is permitted. And all this does not apply to Parashat Zachor and Parashat Parah which must be read since they [require a quorum of] ten [people] as [obligations] from the Torah that require one to concentrate and hear [them] from the mouth of the reader. And it is proper that for all the parshiot [throughout the year], it is fitting to be scrupulous with the reader's words and for one to pay attention and hear them from the reader.

English

It is forbidden to speak when the Maftir is reading from the Prophets until he finishes, just like with the Torah scroll.

English

There is no need to stand up during the reading of the Torah. (There are those who are stringent and stand, and that was the practice of the Mahara"m). (Mordechai - chapter "Rabbi Eliezer d'Milah")