ideally we want to be the fly on the wall, observing without interfering... but if the subjects become aware of their role, it's imperative that the dialog about the nature of the experiment be conditioned in such a way as to maintain the integrity of the dependent and independent variables that self-awareness would destroy. Hence slightly change the narrative and focus, instead of telling a white lie by withholding information (which is normally the case), add a layer of irrelevant information so the attention shifts away from the experiment and element of awareness of their role is eliminited due to following a slightly deviated route of action.