We discuss the mechanism(s) of bar formation in isolated and tidally interacting disc galaxies using the results of idealized collision-less N-body simulations of the galaxies. In order to better understand the mechanism, we investigate orbital eccentricities (e), epochs of apocentre passages (ta), azimuthal angles at ta (φa), precession rates (Ωpre), for individual stars, as well as bar strengths represented by relative m = 2 Fourier amplitude (A2) and bar pattern speeds (Ωbar). The main results are as follows. A significant fraction of stars with initially different φa and Ωpre in an isolated disc galaxy can have similar values within several dynamical time-scales. This synchronization of φa and Ωpre, which is referred to as apsidal precession synchronization ('APS') in this study, is caused by the enhanced strength of the tangential component of gravitational force. A weak seed bar (A2 < 0.1) is first formed through APS in local regions of a disc, then the bar grows due to APS. In the bar growth phase (0.1 < A2 < 0.4), APS can proceed more efficiently due to stronger tangential force from the bar so that it can enhance the bar strength further. This positive feedback loop in APS is the key physical mechanism of bar growth in isolated stellar discs. Bar formation can be severely suppressed in discs with lower disc mass fractions and/or higher Q parameters due to much less efficient APS. APS proceeds more rapidly and more efficiently due to strong tidal perturbation in the formation of tidal bars compared to spontaneous bar formation.