In A Tale of Two Cities, what personal service does Carton do for Darnay and how might this be an example of foreshadowing?
Recognizing the similarity of his physical appearance to that of the man on trial named Charles Darnay, Sydney Carton scribbles a note, crumples the paper, and tosses it to the prisoner's counsel. This counsel, Mr. Stryver, then points to the strong resemblance between the two men in order to create doubt in the witness about the identity of Darnay.
In Book the Second, Chapter III, Charles Darnay, who has ridden on the Dover coach with Miss Manette, her father, Dr. Manette, and Mr. Lorry, has been arrested and charged with treason. One witness who is called to testify states that he saw Darnay engaged in suspicious business at a hotel in a “garrison-and-dockyard town.” With such damaging testimony hanging over him, it seems that Darnay will certainly be found guilty. But, because he resembles Sydney Carton, a legal partner of Mr. Stryver, the witness for the prosecution is unable to make an unequivocal identification of this man who is on trial. Therefore, Charles Darnay goes free.
After the trial, Mr. Carton remarks to Mr. Darnay,
"This is a strange chance that throws you and me together. This must be a strange night to you, standing alone here with your counterpart on these street-stones?"
Carton's question about their resemblance to one another acts as an introduction to Dickens's motif of doubles in A Tale of Two Cities. And because Sydney Carton resembles Charles Darnay and can double for him, there is, indeed, a foreshadowing of another time when Carton will save Darnay because of their resemblance to one another. In the final chapter of Dickens's powerful and moving narrative, Sydney Carton sacrifices himself by taking the place of Charles Darnay (formerly Evremonde), an aristocrat condemned to the guillotine during the French Revolution.
The service your question refers to occurs in Book the Second, Chapter Three, which describes the trial of Charles Darnay for treason. At one stage of the proceedings, the conviction of Darnay is dependent upon his identification by one witness. What Carton notices is that both he and Darnay have a very similar physical appearance, and pointing this out means that Stryver is able to successfully show the witness to be unclear about his positive identification of Darnay:
The upshot of which was, to smash this witness like a crockery vessel, and shiver his part of the case to useless lumber.
What is key to focus on is the way that it is Carton's similarity with Darnay that rescues him from being falsely convicted of treason. This of course is used to foreshadow the ending of this tremendous Dickensian classic, when their likeness is again used to save Darnay from wrongful punishment, but only by Carton sacrificing himself in Darnay's stead.