Always loved that old bald dude in the white hoodie....in the final product, Killian clinks drinks with him and the dude just seems honored/enamored that Hook chose to clink drinks with him.He seemed to really get into the second verse bit anyway.
This was more of a blocking/choreography rehearsal. Since he also had another song with Jennifer Morrison and the rest of the cast to sing too, they wouldn't have him singing live during every rehearsal to learn the choreography.