When Stephen is talking to Susan and the Virologist near the beginning of the film, a doctor can be seen running down the stairs behind him towards the floor below. The camera cuts away and then back to Stephen, showing the doctor running back down the stairs again, then after a second cut back, he vanishes.
Susan: [narrating]
There was darkness. There is light. There are men and women. There's food. There are restaurants. Disease. There's work. Traffic. The days as we know them, the world as we imagine the world.
Susan: It's dark now. But they feel each others' breath. And they know all they need to know. They kiss. And they feel each others' tears on their cheeks. And if there had been anybody left to see them, then they would look like normal lovers, caressing each others' faces, bodies close together, eyes closed, oblivious to the world around them. Because that is how life goes on. Like that.