In A Tale of Two Cities, what did Carton do to ensure that nobody would suspect that a switch had taken place?

Near the end of A Tale of Two Cities (book 3, chapter 13), Sydney Carton enters the Conciergerie, the prison where Charles Darnay and the others awaiting the guillotine are being kept. He has extorted the cooperation of Barsad, who has sneaked him into the prison; he also brings the necessary potions that he bought earlier. Telling Darnay of Lucie’s wishes, he first gets Darnay to switch boots with him, despite his dubious attitude. After Darnay writes his statement, again at Carton’s urging, Carton drugs him and, while he is out, changes clothes with him. He further secures Barsad’s assistance in removing the unconscious Darnay from the prison; outside, the carriage that Lorry had procured is waiting to whisk him away.