Before he dies, Yitzchak desires to give Eisav a Berachah, but when Rivkah finds out about this she decides to help Yaakov trick Yitzchak into giving him the Berachah instead. Yaakov is concerned that his father will find out his true identity when he feels his skin and realizes that he is not hairy like Eisav is. Why is this Yaakov’s only concern? Why isn't he concerned that he won’t sound like Eisav or that he won’t smell like Eisav?
The Beit HaLeivi answers that before Eisav left to go hunting, he anticipated that Yaakov would try and trick his father into blessing him. Eisav anticipated that Yaakov would try to mimic Eisav’s voice as part of the trick, so Eisav decided that he would show that he was himself by talking in Yaakov’s voice. Yaakov, being aware how Eisav’s mind worked, knew that all he had to do was to feel like Eisav, because he knew that his father was expecting someone to come in who talked like Yaakov. The Midrash explains that he was even able to mimic his brother’s smell because Rivkah had stolen Eisav’s hunting coat, allowing Yaakov to smell like Eisav. This explains why all Yitzchak wanted to do was feel Yaakov before he blessed him because the two things that he was expecting from Eisav were hairy arms and Yaakov’s intonation. Even if one thinks he can outsmart the Ratzon Hashem it will always work out and in this case it even backfired!