What effect does Dickens achieve by telling of the tranquil life of the Darnay and the storming of the Bastille in the same chapter? Why do you think Dickens again refers to Lucie hearing footsteps at the end of Chapter 21?

Any time an author can show some kind of contrast his point in the writing is more impressive. To see the chaos of the revolution and the storming of the Bastille against the calm life of some French citizens before the revolution began shows us how really violent and out of control the storming was. Mentioning Lucie hearing some footsteps helps build suspense; we want to know who is the owner of those footsteps and what effect that person is going to have on the story.