In A Tale of Two Cities, what personal service does Carton do for Darnay and how might this be an example of foreshadowing?

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.