We do not recommend that a NDA be signed before the first interview with the experts. Indeed, signing such an agreement often takes a lot of time and is not necessary at least for the first interview. However, before and during the interview, it is necessary to make sure of the level of information communicated on both sides, not to disclose any confidential information, and to suggest to the expert to do the same.
The points raised during the first interview - description of the approach, presentation of the subject, discussion of the expert's environment, initial discussion of his/her possible contribution - may remain sufficiently general not to require the signing of a non-disclosure agreement. This first interview allows, above all, to validate the appropriateness of subsequent discussions: the interest of a collaboration with the expert, his/her openness to such a collaboration and the associated material conditions.
The signature of the non-disclosure agreement can be done during this second stage if necessary.