The word interrogation may give one the visual image of something akin to a police interrogation; but in a recent NLRB decision, the NLRB held that when a non-union shop subcontractor asked individual union member workers about their union affiliation in discussions about hiring those workers, the subcontractor violated the National Labor Relations Act. That decision is Euro Buildings, Ltd and International Union of Bricklayers & Allied Craftsworkers, Ohio-Kentucky Administrative Council, Local 22 Ohio, 2014 WL 5410010 (N.L.R.B. Div. of Judges 2014, adopted 2014 WL 6969677 (N.L.R.B. 2014).
The NLRB discussed that whether discussions amounted to interrogation required the NLRB to consider whether all of the associated circumstances of the discussions reasonably tended to evidence restraint, coercion, or interference with rights guaranteed by the Act. The NLRB identified related factors as including the identity of the questioner, the place and method of the questions, the background of the questioning, the nature of the information sought, and whether the employee is an open union supporter.
For that case, the NLRB judge found that the non-union subcontractor's questions of the applicants about their union affiliation and membership was clearly coercive in nature and reasonably tended to coerce the applicants, and as such interfered with their rights under the Act. His decision seems to suggest any inquiry during the interview process about affiliation would similarly qualify as an coercive interrogation - the indirect threat being if you say you are affiliated as an applicant then you can't get the job.