The two girls on the phone never actually talked to one another. Daniella had to watch the footage of Rebekah talking on her computer and time her lines to make it seem like they're having a conversation. She had to get it perfect because there was no cuts as it was a split screened shot.
Rebekah had very little time to look over her lines or to shoot. So most of her acting was improv, loosely based on the script and direction given from the director.
The writer came up with the story based on a personal experience of an eye surgery gone bad that left him wearing an eye patch for almost two months. He couldn't drive a car because of the lack of depth perception.