  MAXI J1820+070 (optical counterpart ASASSN-18ey) is a black hole candidate
discovered through its recent very bright outburst. The low extinction column
and long duration at high flux allow detailed measurements of the accretion
process to be made. In this work, we compare the evolution of X-ray spectral
and timing properties through the initial hard state of the outburst. We show
that the inner accretion disc, as measured by relativistic reflection, remains
steady throughout this period of the outburst. Nevertheless, subtle spectral
variability is observed, which is well explained by a change in coronal
geometry. However, characteristic features of the temporal variability -
low-frequency roll-over and QPO frequency - increase drastically in frequency,
as the outburst proceeds. This suggests that the variability timescales are
governed by coronal conditions rather than solely by the inner disc radius. We
also find a strong correlation between X-ray luminosity and coronal
temperature. This can be explained by electron pair production with a changing
effective radius and a non-thermal electron fraction of ~20%.
