She works as a part-time actress and is a stand-in.
Actually, "stand-ins" are different from "body doubles." A body double is actually seen on-screen. They are used in camera shots where, for example, the star does not want to appear nude or perhaps the star's body isn't svelte enough for the director's purpose. (For example, in the film Pretty Woman, the opening shots that show Julia Roberts getting out of bed and pulling her stockings on and then getting dressed are actually close-up shots of a body double.)
A stand-in is a person used on a film or TV set to enable the director and other crew members to set the lighting and camera angles perfectly before the actual actor/actress comes on set. (For example, due to child labor laws the kids that starred on The Brady Bunch were limited to how many hours they could work. So the producers used dwarves as stand-ins for the kids whenever possible to set up the lighting and such before actually filming a scene.
Another example of a body-double (relating to an old film, but that I heard about earlier this week) is the shower scene in Psycho. When you can see the face of the star in question, it's usually a head-shot. Anything more revealing (the navel, for example) was done with a body-double.