Real-time observables, such as particle decay widths, are important for all contexts in which relativistic thermal field theory is being deployed. I will start by introducing the formalism to evaluate such rates perturbatively, explain where difficulties arise, and what progress has been made in recent years. To illustrate how the techniques apply, neutrino dynamics in the early universe will be considered—a topic for which it has become timely to achieve well-controlled theoretical computations within the Standard Model. Specifically, I will outline a recent study of QED corrections to the neutrino interaction rate at decoupling temperatures.