The respondent interview is where an investigation is most often criticised, and the criticisms are usually about fairness rather than about the findings.
Adequate notice of the allegations. Enough detail to respond meaningfully. Confronting someone with a general accusation and asking for their side is not a fair process, and the account obtained is worthless.
Timing. After the documents are understood and the complainant and witnesses have been interviewed, so that the account can be tested.
Warnings. The Upjohn warning where counsel conducts the interview. Representation rights where a collective agreement or policy provides them.
Put the evidence. Each specific allegation, and the material contradicting their account, so they can address it. Findings made on evidence never put to the respondent are routinely overturned.
Ask for their witnesses and documents, and follow up on them. Failing to pursue exculpatory leads is the most common substantive criticism.
Do not promise outcomes or characterise the likely consequence.
Second interview. Where new information emerges, return to the respondent rather than making a finding on material they never saw.
Record. A memorandum, and where the account changes across interviews, a note of what changed and when.