This thesis advances the understanding of charge propagation in a classical background through the study of the canonical quantisation of a charged particle in an intense laser background and the associated twopoint function. We will clarify how the causal propagator can be extracted. In addition, in this thesis we will provide an interpretation of this propagator in terms of degenerate quantum processes. We show how to use the Feynman diagram technique to build up a perturbative description of the interaction with the laser background. This also includes how multi-pole structures emerge as a perturbative feature. For all orders in perturbation theory we calculate the associated multiple wave-function renormalisations. We use Bessel functions to describe the distortion of the plane wave solutions due to the interaction with the laser. This is done for both a circularly and a linearly polarised laser.