  The behaviour of oscillating scalar spectator fields after inflation depends
on the thermal background produced by inflaton decay. Resonant decay of the
spectator is often blocked by large induced thermal masses. We account for the
finite decay width of the inflaton and the protracted build-up of the thermal
bath to determine the early evolution of a homogeneous spectator field, \sigma,
coupled to the Higgs Boson, \Phi, through the term g^2 \sigma^2 \Phi^2, the
only renormalisable coupling of a new scalar to the Standard Model. We find
that for very large higgs-spectator coupling g > 10^{-3}, the resonance is not
always blocked as was previously suggested. As a consequence, the oscillating
spectator can decay quickly. For other parameter values, we find that although
qualitative features of the thermal blocking still hold, the dynamics are
altered compared to the instant decay case. These findings are important for
curvaton models, where the oscillating field must be relatively long lived in
order to produce the curvature perturbation. They are also relevant for other
spectator fields, which must decay sufficiently early to avoid spoiling the
predictions of baryogenesis and nucleosynthesis.
