We define and characterize localized causal operations in a real scalar quantum field theory, and we show that these operations exactly correspond to those satisfying various equivalent no-signalling conditions proposed in previous works. The simplicity of this characterization allows us to define a faithful quantifier of how much a map enables faster-than-light signalling in a given field state. Finally, we apply our results to revisit relevant examples, providing straightforward proofs of their causal (or acausal) behaviour.