Definition:
Undergoer that is central to an event or state that does not have control over the way the event occurs, is not structurally changed by the event, and/or is characterized as being in a certain position or condition throughout the state.
Example:
THE BALL rolled down the hill.
Theme
Definition:
The starting point (possibly metaphoric) of an action; it exists independently of the event.
Example:
I gleaned FROM THE EVIDENCE how she did it.
Source