1) Halachah forbids making any kind of interruption in between the placing of the tefillin shel yad and the tefillin shel rosh. One may not speak even a single word in any language — including Hebrew — in between the laying of the two tefillin, even for the purpose of a mitzvah, such as to return a greeting to a Rabbi.[55]

2) If a person did make an interruption, even accidentally, before donning the tefillin shel rosh, he must recite the berachah ברוך אתה ה' אלקינו מלך העולם אשר קדשנו במצוותיו וציונו על מצוות תפילין .[56] (The word מצות is pronounced in the singular form — mitzvat — and not in the plural form — mitzvot.[57]) This berachah is recited after one places the strap of the tefillin shel rosh on his head, but before he fastens it around his head.[58] One must ensure that his head is covered while reciting this berachah, like with all berachot.[59] 

3) One should ensure not to make even a silent pause between the placing of the tefillin shel yad and tefillin shel rosh, even if he keeps his mind focused on the tefillin during the pause. The Torah writes in introducing the obligation of tefillin, והיה לך לאות על ידך ולזכרון בין עיניך (“It shall be as a sign upon your arm and as a reminder between your eyes” — Shemot 13:9). Rather than allocating an independent clause for each tefillin (“It shall be. . .upon your arm; it shall be. . . between your eyes”), the Torah incorporates both into a single phrase (“It shall be. . .upon your arm and. . .between your eyes”), implying that one should put them on together, without pausing in between. Nevertheless, if one did make a silent pause, he does not recite a berachah before donning the tefillin shel rosh.[60]

a. One should therefore ensure that the tefillin shel rosh is in front of him and readily accessible as he begins placing the tefillin shel yad.[61]

b. It is likewise forbidden to signal to somebody with hand motions, facial expressions and the like in between the placing of the tefillin shel yad and tefillin shel rosh, except for the purpose of a mitzvah. Thus, for example, if a poor person approaches somebody as he dons his tefillin, the person may not give the poor man charity but he may signal to him that he should wait.[62]

c. One may kiss his tefillin shel rosh before putting it on his head, as this does not constitute an interruption.[63]

4) Even if one fulfilled his berachah obligation by listening to the berachah recited by somebody else, he may not speak until after he has placed his tefillin shel rosh, and, if he did speak, he must recite the aforementioned berachah before donning the tefillin shel rosh. If the person who recited the berachah speaks before donning his tefillin shel rosh, then he must recite the berachah before wearing the tefillin shel rosh, but the one who listened to his berachah is not required to recite a new berachah before placing his shel rosh.[64 ]

5) Preferably, one should not make even interruptions that are somehow relevant to the tefillin. Where this is absolutely necessary, however, one may interrupt, and in such a case he does not recite a new berachah over the tefillin shel rosh, even if he spoke after reciting the berachah and before fastening the tefillin shel yad around his arm.[65]

6) One may not interrupt while laying tefillin even to respond to kaddish, kedushah or barechu. If one hears kaddish, kedushah or barechu in between the laying of the tefillin shel yad and the tefillin shel rosh, he should remain silent and think the response in his mind. Nevertheless, if one did verbally respond, he is not required to recite a new berachah over the tefillin shel rosh.[66]

7) If, after reciting the berachah over the tefillin, one realizes that the congregation will soon recite kaddish, kedushah or barechu, he may place the tefillin shel rosh before wrapping the strap of the shel yad seven times around his forearm, so that he will be able to respond together with the congregation. He should firmly fasten the shel yad on his arm such that it stays in place — which might require wrapping the straps one or two times around the arm — don the shel rosh, and then respond to the kaddish, kedushah or barechu. Thereafter, he should complete wrapping the strap seven times around his arm.[67]

8) It is preferable not to bring down one’s sleeve to cover the tefillin shel yad until after he places the tefillin shel rosh, as doing so may constitute a hefsek in between the laying of the shel yad and the shel rosh.[68]

9) One who wears tefillin Rabbenu Tam may respond to kaddish or kedushah as he puts on the tefillin, though, in such a case, he should move the tefillin shel yad a bit before donning the shel rosh in order that they be considered to have been donned together, rather than on two separate occasions.[69]