It occasionally happens when a group of people begin a meal together (like at a Se’uda Shelishit in the synagogue) that a person hears somebody else recite the Beracha of "Al Netilat Yadayim" as he washes his hands. The question arises as to whether one should answer "Amen" to a Beracha which he hears after he washed his hands but before he dried them and recited the Beracha. Would answering "Amen" at this point constitute a Hefsek (inappropriate interruption) in the middle of the performance of the Misva?
Hacham Ben Sion Abba Shaul (Israel, 1923-1998), in his work Or Le’sion (vol. 2, 11:3), rules that one may answer "Amen" in such a case. In his view, the washing constitutes the primary Misva, and thus once a person finishes washing his hands – even though he has not dried them – he has completed the Misva. In fact, Hacham Ben Sion writes, we should, theoretically, recite the Beracha of "Al Netilat Yadayim" before washing our hands. The reason we recite the Beracha only after washing is because occasionally our hands are dirty before we wash them, and it is forbidden to recite a Beracha with soiled hands. Fundamentally, however, we should recite the Beracha before washing, because washing constitutes the essential Misva, and the Beracha is always recited before the performance of a Misva. Therefore, Hacham Ben Sion claims, it is permissible to answer "Amen" in between the washing and drying of one’s hands, since the Misva has already been completed after the washing.
Hacham Ovadia Yosef, however, in his work Yabia Omer (vol. 8, p. 243), disagrees. He notes that according to several authorities – including Tosafot, the Rosh, the Mordechi, the Samag, the Meiri and the Tur, drying is part and parcel of the Misva of Netilat Yadayim. And it is for this reason, according to this view, that we recite the Beracha only after drying our hands – because we do not complete the Misva until after the drying. Therefore, one may not interrupt in between the washing and the drying, as this would be considered a "Hefsek" in the middle of the Misva. This is the position that one should follow.
It should be emphasized that it is forbidden to speak once one has begun washing his hands. Many people mistakenly think that it is permissible to speak after one has begun washing, before he recites the Beracha. This is incorrect. It is forbidden to speak from the moment the first drops of water come in contact with one’s hand with the first pouring over the right hand.
Summary: Once a person begins Netilat Yadayim, it is forbidden to speak, from the moment the water touches his hand with the first pouring. He may not answer "Amen" to a Beracha until after he dries his hands.