  Growth processes and interface fluctuations can be studied through the
properties of global quantities. We here discuss a global quantity that not
only captures better the roughness of an interface than the widely studied
surface width, but that is also directly conjugate to an experimentally
accessible parameter, thereby allowing us to study in a consistent way the
global response of the system to a global change of external conditions.
Exploiting the full analyticity of the linear Edwards-Wilkinson and
Mullins-Herring equations, we study in detail various two-time functions
related to that quantity. This quantity fulfills the fluctuation-dissipation
theorem when considering steady-state equilibrium fluctuations.
