A method of fastening a wall panel to a wall, according to one illustrative embodiment, comprises coupling a retaining body to a mounting body mounted on an outer surface of the wall adjacent a side edge of the wall panel. Coupling the retaining body to the mounting body may comprise coupling a first connector on an inner surface of the retaining body to a second connector on the mounting body, and positioning a retainer on the retaining body against a retaining surface on the wall panel to fasten the wall panel to the wall. Kits and wall assemblies according to other illustrative embodiments are also disclosed.