As a Brachat HaMitzvah it must be said Over LeAsiyatan, prior to the action of the mitzvah. 
One may not make any interruptions between the Bracha and the actual Mitzvah. Nonetheless, an action that's necessary to do the Mitzvah isn't considered an interruption between the Bracha and Mitzvah. For example, after making the bracha on Chanukah candles it's not an interruption to get candles or a match because these are necessary elements for the Mitzvah. 
When saying a Brachat HaMitzvah, preferably one should stand, however, after the fact if one said the bracha in any position one fulfilled one's obligation.
Once one started the mitzvah but didn't complete it one should still not make an interruption but if he does he doesn't recite another bracha. For example, if one made the bracha for bedikat chametz and started to check the house and then spoke he doesn't have to recite another bracha since he started the mitzvah.
If one spoke between the bracha and the mitzvah one needs to make another bracha.
If one spoke between multiple mitzvot it is considered an interruption and one should recite another bracha on the next mitzvah. For example, if someone is putting up mezuzot in his house and his intention was to put up all of the mezuzot if he spoke between one and the other one should recite another bracha.
↑Mishna Brurah 8:2 and 585:1. The Avudraham (Birchot Hashachar s.v. keshemitatef) writes that for 6 brachot one should stand: putting on Counting the Omer, Birchat Halevana, Tzitzit, Shofar, Lulav, and Milah. He says that there's a gezerah shava of lachem between each of these cases and counting the omer where there is a derivation from the pasuk to imply that it should be said standing. He adds that some say that if the mitzvah doesn't involve getting benefit the relevant bracha should be said standing. The same appears in the Kol Bo quoting a Yerushalmi. See Pri Megadim M"Z 432:3. Another example is mezuzah that the bracha should be said standing (Mezuzah Vhilchoteha 16:2, see Rivevot Efraim 2:29:6).
↑Gemara Brachot 44b. Tosfot s.v. velebani says that according to everyone there’s no bracha achrona for mitzvot besides for Tefillin. Even Tefillin only those in Israel made a bracha achrona because they held that you can’t wear tefillin at night mdeoritta, however, we don’t hold like that opinion. Rashba agrees. Ramban Niddah 51b s.v. ha argues that there’s a bracha achrona for any mitzvah that takes time. That’s only for those of Israel who we don’t follow.
↑Rosh Chullin 6:6 writes that unlike if one made an interruption between two mitzvot such as between putting on the two Tefillin or doing two shechitot if one just started a mitzvah and didn't complete it an interruption doesn't require the person to recite another bracha. Example include Shofar, Hallel, and Megillah. Since once one started a mitzvah one has an obligation to finish it even if one makes an interruption that doesn't completely cause one to despair from having the bracha apply to the whole mitzvah. But if there are multiple mitzvot and one can take a break at any time creating an interruption indeed causes a person to consider the bracha as having terminated. This is quoted by Bet Yosef YD 19:5. The Rashba responsa 1:244 also applies this to Shofar, Hallel, and Megillah that once one starts the mitzvah no interruption will cause one to require a new bracha. Rama 690:5 quotes this. Tosfot Pesachim 115a s.v. matkif agrees. Shulchan Aruch 475:2 quotes this. See Or Letzion 1:39.
↑Shulchan Aruch 25:9 with respect to Tefillin writes that if a person speaks between the bracha and the mitzvah he needs another bracha. Similarly, Shulchan Aruch 167:6 with respect to brachot on food writes that if one spoke about something unnecessary to the bracha one needs to recite another bracha. Mezuzah Vhilchoteha 16:12 writes that the same is true of the bracha before putting up a mezuzah that one would require another bracha if one spoke in between.
↑The Tosfot Chullin 87a s.v. umichsi discuss whether speaking between several acts of shechita would require a person to make another bracha for shechita. He says on the one hand we find that for Tefillin speaking between putting on the Shel Yad and Shel Rosh requires another bracha, while speaking in a meal after a bracha doesn't require one to make another bracha in the middle of the meal. He suggests that perhaps shechita is similar to a meal and dissimilar to the Tefillin since the Tefillin is one mitzvah, whereas each Shechita is independant. Rav Soloveitchik (Reshimot Shiurim Brachot 11b s.v. vheneh) explained this approach as follows: the bracha only needs to apply to the first act of the mitzvah and everything else is exempt. For food and shechita only the first bite or first shechita requires a bracha and everything else is exempt even if there's talking in between. Tefillin however are a unit and the bracha applies to both together and speaking in between one and the other creates an interruption before the first act was complete. See Divrei Yirmiyahu Tefillin 4:6 who suggests another explanation in Tosfot. The Smag 64 quotes the Sefer Hatrumah 39 who was also in doubt about this question but quotes the Bahag who writes that one needs to recite another bracha. The Bet Yosef YD 19:5 cites the Ran Chullin 28b who quotes the Rif Rosh Hashana 11a, Raavad, and Ramban Chullin 86b s.v. modeh who agree with the Bahag unlike the Rabbenu Yonah who holds otherwise. Rashba in Torat Habayit 21a and Rosh Chullin 6:6 agree with Bahag. Shulchan Aruch 19:5 rules like the Bahag to recite another bracha. Hamezuzah Vehiloteha 16:15 applies this discussion to putting up several mezuzot in the house that if one speaks between the mitzvot one should recite another bracha. He cites the Chovat Hadar p. 79 who agrees.