I'm using a Negative Binomial model to analyses the effect of a scale (NR) and another binary variable (Condition) on a count outcome. When I add both variables to the model as main effect, they are not significant. However, when I add the interaction (Condition:NR) to the model, they become significant. I hear this is a common phenomenon but I'm not sure what steps to take to interpret it.
Also, are there any further tests I can use to interpret the interaction? Because the Negative Binomial outputs don't say much overall.