The Tale of 2 Cities part 3

1. In Book 1, Chapter 3, the author says:

A wonderful fact to reflect upon, that every human creature is constituted to be that profound secret and mystery to every other. A solemn consideration, when I enter a great city by night, that every one of those darkly clustered houses encloses its own secret; that every room in every one of them encloses its own secret; that every beating heart in the hundreds of thousands of breasts there is, in some of its imaginings, a secret to the heart nearest it!

One of the themes in the novel involves secrets. Pick two characters and explain the role of their secrets in the novel.