Chlorine nuclear quadrupole resonance absorption lineshapes are reported for powder and single-crystal samples of the antifluorite K₂OsCl₆ for a range of temperatures above the cubic to tetragonal phase transition. The temperature dependence of the lineshape and its integrated intensity, as well as the isotope independence of the results, provide clear evidence of precursor effects associated with nonlinear anharmonic behaviour. The results suggest that the detailed dynamics associated with the precursor clusters is sensitive to whether point defects or dislocations are dominant as the primary cause of line broadening.