[Foster is startled by the knock--less because of the knock and more because sudden noises always startle him. He assumes, reasonably, that whoever it is came for his roommate and not him, so when he approaches the door, he's bracing himself not to receive an actual visitor, but to apologise for the surely crushing disappointment on the other end when they discover that he is the only one here.
When he opens it, though... he stares, blankly, for about an entire second.]
Oh.
[His eyelids drop a little, and his voice becomes audibly more detached.]
no subject
When he opens it, though... he stares, blankly, for about an entire second.]
Oh.
[His eyelids drop a little, and his voice becomes audibly more detached.]
It's you.