Gemara

(Mishnah - R. Yosi ha'Glili): Even if one transgressed a Mitzvah mid'Rabanan, he returns (with fearful people) and does not fight in war.

(Beraisa): If one talked in between putting on the Tefilin Shel Yad (of the hand) and the Shel Rosh (of the head), this is an Aveirah, and he returns.

This is like R. Yosi ha'Glili.

Menachos 36a (Rav Chisda): If one talked in between Tefilin Shel Yad and Shel Rosh, he returns and blesses again.

Rishonim

Rambam (Hilchos Tefilin 4:6): If one blessed 'Lehani'ach Tefilin' and tied the hand Tefilin, he may not talk before putting on the head Tefilin, even to return Shalom to his Rebbi. If he talked, this is an Aveirah.

Kesef Mishneh: Tana'im argue only about whether or not one returns from war for this, but all agree that it is an Aveirah.

Rosh (Hilchos Tefilin Siman 15): One may not interrupt even for Amen Yehei Shmei Raba. Old Hilchos Tefilin say that if one interrupted with secular talk, he returns from war. If he answered Kedushah or Amen Yehei Shmei Raba, he does not return from war, but he blesses twice on Shel Rosh, for each Tefilah needs two Berachos. Even though he does not return from war, one may not cause an unnecessary Berachah. On Yom Kipur, the Kohen Gadol reads some verses by heart. They do not bring another Sefer, to avoid an extra Berachah.

Tosfos (Chulin 87a DH u'Mechasi): It is not clear whether or not talking between Shechitos obligates another Berachah. We cannot learn from Tefilin, for it is one Mitzvah, and he should not interrupt it.

Terumas ha'Deshen (2:117, cited in Beis Yosef OC 25 DH Kasuv b'Kisvei): It is good to put on Tefilin during Chol ha'Mo'ed without a Berachah. Even so, one may not talk in between. This is called an Aveirah not only because he causes an extra Berachah. (Tosfos connotes that even if causing a new Berachah were permitted, an interruption in Tefilin would be forbidden.)

Mordechai (Hilchos Ketanos 12a): R. Tam says that if one interrupted to answer Kaddish or Borchu, this is not a Hefsek, for he cannot put them on until answering with the Tzibur.

Rashi (Menachos 36a DH Sach): If he spoke in between and did not repeat the Berachah, this is an Aveirah for Isur he returns from war.

Tosfos (Menachos 36a DH Sach and DH Aveirah): Rashi connotes that if he repeated the Berachah, he did not sin. This is wrong. One may not cause an unnecessary Berachah!

Poskim

Shulchan Aruch (25:9): One may not interrupt by talking in between putting on the Shel Yad and the Shel Rosh.

Mishnah Berurah (28): One may not interrupt even for a Mitzvah, e.g. to return Shalom to his Rebbi. Even if one does not bless on Tefilin, e.g. one who wears Tefilin during Chol ha'Mo'ed, R. Tam Tefilin, or he removed them with intent to wear them again, it is an Aveirah to interrupt.

Mishnah Berurah (29): L'Chatchilah, one may not interrupt unnecessarily for a long time even without speaking, even if he does not divert his mind. One should be careful not to wink or gesture. If he puts on the Shel Rosh in a different house, this is like blessing on a Talis Katan at home and putting on a Talis Gadol in the Beis ha'Keneses (8:13. Most say that this is a Hefsek. The Chayei Adam is concerned for the other opinion, and says not to bless again unless the Beis ha'Keneses was far away, he did not plan to exempt both when he blessed, or he ceased to think about it in the middle.)

Hilchos Ketanos (1:57): Even winking or gesturing is a Hefsek of one's mind from the Mitzvah, and obligates an extra Berachah.

Kaf ha'Chayim (57): All the Acharonim say (b'Di'eved) not to bless again.

Rema: We normally bless two Berachos even if he did not interrupt. Therefore, if he interrupted, he blesses on the head Tefilin 'Lehani'ach' and also 'Al Mitzvas.

R. Akiva Eiger (DH Asur): Panim Me'iros (1:59) says that if in between one heard someone else bless Lehani'ach Tefilin, answering to the same Berachah itself is not an interruption. Devar Shmuel (59) disagrees. I also disagree, for the Tur (Siman 61) permits answering Amen to the Berachah before Shema because it is no worse than greeting someone (which is permitted between Berachos of Shema - Berachos 13a). If not, it would be a Hefsek! The Shulchan Aruch (66:7) forbids answering Amen to Go'al Yisrael before Shemoneh Esre. One opinion permits, but even he agrees that it is a Hefsek from the Berachah. He considers the Amen to be part of Ge'ulah (redemption), so Shemoneh Esre is adjacent to Ge'ulah.

Shulchan Aruch (10): If one interrupted for needs of Tefilin, he does not bless again.

Magen Avraham (16): L'Chatchilah, one may not interrupt even for needs of Tefilin, unless there is no alternative.

Mishnah Berurah (33): The same applies if he interrupted between Lehani'ach and putting on the Shel Yad.

Eshel Avraham (16): Why didn't the Shulchan Aruch teach this bigger Chidush? It seems that he wanted to teach, by inference, that one may not interrupt even between the Shel Yad and the Shel Rosh.

Shulchan Aruch (ibid.): If he heard Kaddish or Kedushah in the middle, he should not answer. He listens and intends for what they say.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): The Rosh forbids answering. The Mordechai holds that one must answer, and he need not bless again. The Agur permits answering, but then one must bless again. Even if he only listened he must bless again, for listening is like answering. This is like the opinion that forbids listening to Kaddish or Kedushah during Shemoneh Esre. We hold that listening is like answering to be Yotzei, but not to be considered a Hefsek. Therefore, one may listen.

Mishnah Berurah (36): If one interrupted, he blesses again. (Some disagree.) If one does not bless on Tefilin, e.g. on Chol ha'Mo'ed or R. Tam Tefilin, he may answer (since the only Isur is causing an extra Berachah). He should move the Shel Yad before putting on the Shel Rosh, so both Mitzvos are done together.

Kaf ha'Chayim (63): The Mechaber wrote Stam not to interrupt. This connotes that one does not bless again. We are lenient about Safek Berachos, even though we Sefardim (who do not bless on the Shel Rosh) have another Safek: perhaps the Halachah is that one always blesses on the Shel Rosh.

Kaf ha'Chayim (52): The Levush and Eliyahu Rabah forbid answering even when we do not bless on Tefilin. We may rely on those who permit, since some permit even when we bless on Tefilin! However, one should not interrupt to learn in between.