  We study the attractive Hubbard model in the Lieb lattice to understand the
normal state above the superconducting critical temperature in a flat band
system. We use cluster dynamical mean-field theory to compute the two-particle
susceptibilities with full quantum fluctuations included in the cluster. At
interaction strengths lower than the hopping amplitude, we find that the normal
state on the flat band is an insulator. The insulating behavior stems from the
localization properties of the flat band. A flat-band enhanced pseudogap with a
depleted spectral function arises at larger interactions.
