Laws applicable if Olah or Baal Korei spoke or made an interval between the blessing and reading:[1]

After the Olah recites the first blessing, it is initially forbidden for him speak until after the reading and his recital of the after blessing.[2] [Initially, this applies likewise to the Baal Korei.[3]]

Bedieved if spoke between blessing and reading:[4] If the Olah spoke [of matters unrelated to the reading, even if they are words of Torah[5]] after the blessing, prior to the start of the reading, then he is required to repeat the blessing. [This applies even if he only spoke one word, and applies whether it was said in Lashon Hakodesh or another language.[6] However, if the Baal Korei spoke between the blessing of the Olah and start of his reading, then Bedieved the blessing remains valid.[7]] 

Bedieved if spoke in middle of reading:[8] If he spoke after the Baal Korei began the reading and already read a few verses [or even one verse[9]], then he is not required to repeat the blessing. This applies whether he spoke of mundane matters or of words of Torah.

Bedieved if spoke after the reading, prior to the after blessing:[10] If the Olah spoke after the conclusion of the reading, prior to saying the after blessing, he may nevertheless recite the after blessing. Furthermore, some are accustomed even initially to speak before the after blessing, such as for a Rav to speak Divrei Torah or to make a bid for charity after his Aliyah, prior to the after blessing being said by him.[11] Practically, however, this is not to be done, and if a long interval was made, he is no longer allowed to recite the blessing.[12]

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[1] See Michaber 140:2; Ketzos Hashulchan 25:8; Piskeiy Teshuvos 140:2

[2] See Ledavid Emes 6:61; M”B 140:7; Ketzos Hashulchan 25:8; Piskeiy Teshuvos ibid

[3] See Rav Poalim 3:42; Birchas Habayis 44:26; Rivivos Efraim 1:102; Piskeiy Teshuvos ibid

[4] Implication of Michaber ibid; M”B 140:6; Piskeiy Teshuvos ibid; See Admur 432:6

Other opinions: Some Poskim rule that he does not repeat the blessing even if he spoke between the blessing and reading. [Mur Uketzia 140; Peri Chadash 140; Kaf Hachaim 140:11; See ; Piskeiy Teshuvos ibid footnote 6]

[5] See M”B ibid; Admur ibid

[6] M”B 140:6

[7] Rav Poalim 3:42; Birchas Habayis 44:26; Rivivos Efraim 1:102; Piskeiy Teshuvos ibid

[8] Michaber ibid

[9] M”B 140:6

[10] Ledavid Emes 6:61 [However if is a long Hefsek, then he can no longer say blessing]; Birchas Habayis 44:26; Yabia Omer 1:9; Piskeiy Teshuvos ibid

[11] See Ledavid Emes ibid; Nachlas Yaakov 48

[12] Ledavid Emes ibid

[13] Beis Yehuda 30; Emes Leyaakov 70; Chida in Birkeiy Yosef 139:5 and Ledavid Emes 6:70; Rav Poalim 4:8; Kaf Hachaim 139:36

[14] See M”B 215:1 in name of P”M and M”B 140:6

[15] Shevet Halevi 8:92; Piskeiy Teshuvos 139 footnote 70

The reason: As his elongated Amen can be considered for the need of the reading, in order so the congregation be aware that the reading is About To begin. [Poskim ibid]