  We develop the worldline formalism for computations of composite operators
such as the fluctuation induced energy-momentum tensor. As an example, we use a
fluctuating real scalar field subject to Dirichlet boundary conditions. The
resulting worldline representation can be evaluated by worldline Monte-Carlo
methods in continuous spacetime. We benchmark this worldline numerical
algorithm with the aid of analytically accessible single-plate and
parallel-plate Casimir configurations, providing a detailed analysis of
statistical and systematic errors. The method generalizes straightforwardly to
arbitrary Casimir geometries and general background potentials.
