On the Master's Sun Facebook page they reveal a small preview of what is going to come on episode 13 (it's very interesting too!):
Master's Sun UPDATE!
Episode 13 Written Preview:
"Joong Won has not woken up. Gong Shil feels feels very guilty and blames herself and feels she should leave him. Only if she leaves he will survive. Sitting by his bedside for a few days/weeks, she thinks about the wolf/goat story and feels the wolf who loves more may die, so she left. When Joong Won finally awakes, he forgets about Hee Joo and appears in front of Gong Shil."
Translated by: JulieSean at Soompi Thread
They are filming in the hospital right now (Below is the picture).
Photo for DC Gallery.
From Soompi Thread (bellaangel012):
Possible scene for epi 13 although i cant verify it-
Looks like there is another kiss scene where GS kissed JW and intended to run away but JW grabs her arm and kisses her.
Apparently they get caught by a nurse and he gets reprimanded since he is a patient.
Source: DC Gallery