Answer:
If they are researching something sensitive, such as racism, which is normally frowned upon, the participant is unlikely to act normally around the researcher, as the researcher in a position of authority is able to judge the participant. This means the participant is unlikely to be themselves and so the results will not be valid.
When the participant acts differently towards the researcher because of something this is called impression management.
If the person purposely does something that the researcher wants them to do this is then called demand characteristics.
Safety of respondents and researcher