How do the footsteps in the novel become a larger theme through the novel?
Dickens uses lots of foreshadowing in A Tale of Two Cities. That's not particularly surprising in such a long novel, first of all, but for a work which builds to a Revolution it's actually necessary. The Revolution doesn't just erupt, full-blown, one day. Instead, it moves and grows and shapes and prepares for that fateful day. One of the ways Dickens show this beneath-the-surface movement is the use of footsteps throughout the novel. We read about them (hear them) first at the Manette's house in London (Book 2 chapter 6). It is said that their house was
a wonderful corner for echoes; it had begun to echo so resoundingly to the tread of coming feet, that it seemed as though the very mention of that weary pacing to and fro had set it going.
Notice there is no use of the word footsteps--yet. That comes later in this chapter as well as in subsequent chapters, and it's the sound of a coming Revolution. We hear the footsteps get stronger and louder as the Revolution nears, of course, and they represent the sound of people who will come into their lives, people who will die in the Revolution, and people who will be woven inextricably into their lives in the coming years (like the Dafarges). Book 2 chapter 22 is actually called "Echoing Footsteps," and it opens with a reference to the aforementioned echoing footsteps heard in Dr. Manette's house. The Revolution is at hand. The last chapter of the novel is "The Footsteps Die Out For Ever." The Revolution is no more; instead, it is the beginning of a reconstructed country, one without a king or a dominant aristocratic class.