One way to approach the topic of faces in Roxana is from the perspective of identity and persona. Someone's literal face can hide the person they are inside; on the other hand, someone's face can show their true feelings. There are examples of both in the novel. In your essay, it's not necessary to examine every character in the text. Instead, form a thesis based on your observations of some primary characters, and then write with examples from the text to prove your thesis.
One example of a person's face hiding the person he is inside is Roxana's first husband. He's a handsome person but has no sense or loyalty. Defoe writes,
But this addition to his stock was his ruin, for he had no genius to business, he had no knowledge of his accounts; he bustled a little about it, indeed, at first, and put on a face of business, but he soon grew slack; it was below him to inspect his books, he committed all that to his clerks and book-keepers; and while he found money in cash to pay the maltman and the excise, and put some in his pocket, he was perfectly easy and indolent, let the main chance go how it would.
After the money is gone, he leaves Roxana and their five children. She only finds him again years later, in Paris. Despite being a member of the nobility and a good-looking person, he has proven to be a bad person. On the other hand, many of the servants and poor people in Roxana's life are the ones who come together to find money and help raise her children. The public faces the people present don't adequately show their inner goodness, or lack thereof.
Another area where faces figure into Roxana is how a person's expression can show their true thoughts. At one point, Roxana is speaking to a friend about what she can overhear from two men. Defoe writes,
"Indeed, my good friend," said I, "thou art mistaken this time, for I know very well what they are talking of, but 'tis all about ships and trading affairs." "Well," says she, "then one of them is a man friend of thine, or somewhat is the case; for though thy tongue will not confess it, thy face does."
Roxana admits to the reader that she was going to lie—but then says she's unable to. Her face reveals more truth than the words she speaks. Later, she thinks to herself that she's not sure whether her face will give her away. She thinks,
However, as I have said, her talk made me dreadfully uneasy, and the more when the captain's wife mentioned but the name of Roxana. What my face might do towards betraying me I knew not, because I could not see myself, but my heart beat as if it would have jumped out at my mouth, and my passion was so great, that, for want of vent, I thought I should have burst. In a word, I was in a kind of a silent rage, for the force I was under of restraining my passion was such as I never felt the like of.
Roxana does, however, learn to manage her own face to conceal her thoughts and motives. She thinks at one point that "I did not let the Quaker into any other reason for it than that I was indisposed; and not knowing what other face to put upon that part, I made her believe that I thought I was a-breeding." Earlier, the Quaker was able to read her face. This time, when she sends the Quaker out to do her bidding, the Quaker is fooled by Roxana and does as she wishes because of that false face.
Perhaps that's the point of faces in Roxana. An attractive face can make the public assume you're a good person, but if someone deeply knows another, then that same face makes it difficult to conceal real thoughts and feelings. Having control of one's own face allows a person to control their public persona and better control the situation and people around them. Roxana is so often involved in intrigues and lies that she has to master her own face and manage how she appears to others. If she didn't manage to do so, she never would have amassed the wealth and power she did after her first husband left her.