Coupling localized surface plasmons (LSPs) in Au nanostructure arrays to Fabry-P\'erot cavity modes, both peak splitting and peak locking behaviors can be observed when light is incident normally into the cavity from different directions. These phenomena can be quantitatively described by a model based on modified Fresnel equations, and thus be interpreted as an extended Fano resonance effect. Both the peak splitting and peak locking behaviors arise from the interference between localized surface plasmon resonance and cavity modes with different initial phase difference. When the phase difference of the cavity state and the LSP state is $\ensuremath{\pi}$ or 0, the superposition between a cavity resonance mode and a localized surface plasmon mode can result in the peak splitting or peak locking. Experimental results demonstrate that the separation energy between the split peaks changes with the volumes and densities of the Au nanoparticles and agree with the numerical calculation results quite well.