Makeup, pre-Episode 307.
"ep- 307 "I’m by the steps to help catch stunt guy's head if needed, then feast on the fake intestines/ bbq they used for the scene." 
"ep 307 - this was right before John fell out the door and I was walking to the steps."
She (Sarah) says she gets shot in the head in "Episode 403 ("when the dead come knocking and isolation"), and will send screen shots after she finds them.
Here's what she looks like in real life:
She's primarily a stunt woman, but does some acting - you can read her stunt bio and her fairly extensive TV and movie bio at IMBD IMBD does include her stand-in jobs. She's been a stand-in for Betty White and others.
Not all roles, such as mass Walkers on TWD are credited in TWD episodes.