  Form factors constitute the key building block when organising the
gravitational dynamics in terms of a curvature expansion. They generalise the
concept of momentum-dependent couplings to curved spacetime. Moreover, they may
capture modifications of the gravitational interactions at high energy scales
in a non-perturbative way. We briefly review the role of form factors in
various quantum gravity programmes with special emphasis on non-local,
ghost-free gravity and asymptotically safe gravity. In this context, we
introduce a quantum gravity motivated scalar toy model, which demonstrates that
Lorentzian asymptotic safety may be compatible with the causal propagation of
gravitons.
