Sintering is a very important process in materials science and technological applications. Despite breakthroughs in achieving optimized piezoelectric properties, fundamentals of K$_{0.5}$Na$_{0.5}$NbO$_3$ (KNN) sintering are not yet fully understood, facing densification versus grain growth competition. At present, microscale events during KNN sintering under reducing atmospheres are real-time monitored using a High Temperature-Environmental Scanning Electron Microscope. A two contacting KNN particles model satisfying the Kingery and Berg's bulk diffusion model is reported. Dynamic events like individual grain growth and grain elimination process are explored through a postanalysis of recorded image series. The diffusion coefficient for oxygen vacancies of 10$^{−8}$ cm$^2$ s$^{−1}$ and average boundary mobility of 10$^{−9}$ cm$^4$ J$^{−1}$ s$^{−1}$ are reported for the KNN ceramics. Moreover, the local pore shrinkage is consistent with the Kingery and François's concept of pore stability except that pore curvatures are not all concave, convex or flat due to anisotropic grain-boundary energies. The global grain growth kinetics are described using parabolic and/or cubic laws. The effect of atmospheres and microstructure evolution on the intrinsic and extrinsic contributions to the dielectric response using Rayleigh's law is also explored. These results bring a new breath for KNN sintering studies in order to adapt the sintering process.