To probe the innermost regions of active galactic nuclei (AGN), it is beneficial to use hard X-ray observations as they are less affected by absorption processes. In fact, by analysing the hard X-ray AGN flux variability, one can put strong constraints on the physics of the innermost regions of AGN. According to our current understanding, the primary X-ray emission in radio- quiet AGN originates from corona, a region which is very near to the supermassive black hole (SMBH). Hence, the knowledge of high energy cut-off, Ecut, of primary X-ray continuum is very important as it is often considered as a proxy for the physical properties of X-ray corona. Thus, a combination of variability analysis and spectral studies is essential for our comprehensive understanding of the nature of the X-ray emission processes. In this series of work, using a comparative X-ray flux variability analysis in the 3-79 keV band of a total of 335 AGN from NuSTAR, we have found time variability as small as ~3 minutes. Furthermore, using the spectral data, we have measured different properties of AGN corona including the Ecut and optical depth. A total of 12 Seyfert galaxies from the NuSTAR hard X-ray band (3-79 keV) were analyzed for this purpose. Further details of results from timing and spectral studies and their physical implications will be discussed.