  Coarse-graining atomic displacements in a solid produces both local affine
strains and "non-affine" fluctuations. Here we study the equilibrium dynamics
of these coarse grained quantities to obtain space-time dependent correlation
functions. We show how a subset of these thermally excited, non-affine
fluctuations act as precursors for the nucleation of lattice defects and
suggest how defect probabilities may be altered by an {\it experimentally
realisable} "external" field conjugate to the global non-affinity parameter.
Our results are amenable to verification in experiments on colloidal crystals
using commonly available holographic laser tweezer and video microscopy
techniques, and may lead to simple ways of controlling the defect density of a
colloidal solid.
