In situ high-pressure/high-temperature Raman-scattering analyses on PbTiO3, 0.92PbTiO3-0.08Bi(Zn0.5Ti0.5)O3 and 0.83PbTiO3-0.17Bi(Mg0.5Ti0.5)O3 single crystals reveal an intensity transfer between the fine-structure components of the A1(TO) soft mode. The enhancement of the lowest-energy subpeak, which stems from intrinsic local non-tetragonal polar distortions, along with the suppression of the tetragonal A1(1TO) fundamental mode with increasing pressure and temperature indicates the key role of the local polarization fluctuations in transformation processes and emphasizes the significance of the order-disorder phenomena in both the pressure- and temperature-induced phase transitions of pure PbTiO3 and its solid solutions with complex perovskites. Moreover, the temperature and pressure evolution of the fraction of the local non-tetragonal polar distortions is highly sensitive to the type of B-site substituent.