Proteins bound to lipid bilayer membranes provide a physical model for macromolecular complexes on cellular membranes, and a template for immobilizing molecules at biocompatible surfaces. We study the proteins streptavidin and avidin self-assembled on the surfaces of supported lipid bilayers (SLBs) and fluid lipid bilayer vesicles. Proteins bind to the lipid bilayers through biotin-functionalized lipids, and streptavidin interacts laterally to form two-dimensional protein crystals. We systematically vary the relative amounts of streptavidin and avidin at the lipid bilayer surface to investigate how crystalline proteins influence the structure, fluidity and curvature of lipid bilayers. Our experimental approach combines fluorescence microscopy and X-ray reflectivity, allowing us to determine the molecular structure of the protein/lipid interface, and the influence of microscopic protein crystals on the physical properties of the interface. Using synchrotron X-ray reflectivity, we measure the dimensions of the bound protein layer, as well as the thin hydrated layer separating the proteins from the lipid bilayer. We compare the stability of our protein-coated lipid bilayer interfaces to previous studies of protein-coated lipid monolayers. With fluorescence microscopy, we demonstrate how proteins confer stability to lipid bilayers subjected to mechanical stress and slightly decrease lipid diffusion constants. The experimental framework that we present can be extended to study more complex protein-lipid interactions at a single lipid bilayer interface.