There is an intensity about this meal that sets it apart from every other meal. It seems like every word and action counts. Jesus was troubled in his spirit -- another idiosyncrasy of John's account. And. . . the disciples began to be troubled, too.
Jesus washed their feet and told them they would understand it later. If Jesus does this, no one is above serving! Peter is "all in". He doesn't want Jesus to serve, then he wants to be washed, hands and head. Then he'll lay down his life. However immature and untested his commitment to Jesus, he nonetheless communicates that he wants desperately to follow Jesus.
The disciples didn't understand why Jesus told Judas to do his thing quickly. John records a couple of their guesses. They thought Jesus had sent him on an errand, even though they'd just asked who would betray him.