On the subject of "don't get informal before the formal interview but I'm nervous unless it's informal", why not use two people and clearly delineate their purposes to the interviewee: one person to be informal and get them comfortable before handing them off to another person to conduct a formal interview. If you're up front about the second person and the fact that it will be a formal interview, wouldn't solve both problem to some degree?
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On the subject of "don't get informal before the formal interview but I'm nervous unless it's informal", why not use two people and clearly delineate their purposes to the interviewee: one person to be informal and get them comfortable before handing them off to another person to conduct a formal interview. If you're up front about the second person and the fact that it will be a formal interview, wouldn't solve both problem to some degree?