Do you need a security clearance to conduct contextual inquiries?

To my understanding, a contextual inquiry is conducted in the natural environment of the participant. So for example, if my participant works in an occupation that holds sensititve/proprietary information (e.g. a science lab), how does one go about getting approval properly? I'd like to hear some personal experiences from UX designers so I can get a feel for how it's supposed to be done. Thanks.