When the effect of one factor depends on the level of the other factor. You can use an interaction plot to visualize possible interactions.
Parallel lines in an interaction plot indicate no interaction. The greater the difference in slope between the lines, the higher the degree of interaction. However, the interaction plot doesn't alert you if the interaction is statistically significant.
For example, cereal grains must be dry enough before the packaging process. Lab technicians collect moisture data on grains at several oven times and temperatures.
Interaction plots are most often used to visualize interactions during ANOVA or DOE.
Minitab draws a single interaction plot if you enter two factors, or a matrix of interaction plots if you enter more than two factors.
All of these options let you use fitted means. Usually, plots using fitted values and plots created by the Interaction Plot command using the response will not be the same. They will be the same if the data set is balanced and you fit a full model.