  We review the concept of dynamical ensembles in nonequilibrium statistical
mechanics as specified from an action functional or Lagrangian on spacetime.
There, under local detailed balance, the breaking of time-reversal invariance
is quantified via the entropy flux, and we revisit some of the consequences for
fluctuation and response theory. Frenesy is the time-symmetric part of the
path-space action with respect to a reference process. It collects the variable
quiescence and dynamical activity as function of the system's trajectory, and
as has been introduced under different forms in studies of nonequilibria. We
discuss its various realizations for physically inspired Markov jump and
diffusion processes and why it matters a good deal for nonequilibrium physics.
This review then serves also as an introduction to the exploration of frenetic
contributions in nonequilibrium phenomena.
