Off-shell behaviour of representativeα-α interaction potentials, both local and non-local separable, is compared through the partial wave Kowalski-Noyes half-off-shell functions. Parameters of the existing rank-one separable potentials are redetermined and an additional rank-two potential is constructed for this family. It is found that all these potentials show similar off-shell behaviour for higher partial waves. Their behaviour for low partial waves, however, particularly in the region far away from the energy-shell, is widely different. The off-shell correction for the (α, 2α) reaction at 140 MeV is calculated, as an application, and it is found that separable potentials predict a non-negligible effect.