  Building on a number of previous developments we show that the Diagrammatic
Monte Carlo technique allows one to compute finite temperature response
functions directly on the real-frequency axis within any field-theoretical
formulation of the interacting fermion problem. There are no limitations on the
type and nature of the system's action or whether partial summation and
self-consistent treatment of certain diagram classes are used. By eliminating
the need for numerical analytic continuation from a Matsubara representation,
our scheme opens the door for studies of spectral densities of arbitrary
complexity with controlled accuracy. For illustrative purposes we consider the
problem of the plasmon line-width in a homogeneous electron gas (jellium).
