Well, I'm hoping I understood your question correctly: It might be convenient to try to look at the problem from a regression point of view. In order to test the significance of the interaction term one needs to include both original terms in the analysis before the interaction. In this way one tests the significance of the interaction term when both of the original variables are controlled for. The reason is that interaction in itself is a nonsense multiplicative variable, and it's interpretative meaning stems directly from the fact that influences of both original terms (or main effects) have been partialised from the DV.
Hope this helps.