It's always good to get the input from several people when making a hiring decision, right? Except some opinions are more valuable than others. I recently spoke to a CEO who had to call and apologize to a candidate after one of their less experienced interviewers took things in the wrong direction. That kind of embarrassment you can live without.
As search professionals we always ask who will be involved in the interview sequence. From experience, I can tell you that very few organizations think hard about who to include in the interview sequence until it's already underway. OK, well, that's one way to do it. Here's another...
When you are the hiring manager, figure out who else to include in the interview sequence by separating your additional interviewers into two categories:
Veto Voters: There are "veto vote" people who can derail any hire. These are critical people who must work with your new employee on a regular basis. Veto voters could include peer level colleagues, or sometimes even a Board member. When someone with veto power says no, that person does not get hired. Their opinion really matters in the hiring decision. Ideally, they understand the job, understand the competencies required to succeed in the job, and have a proven track record of making good hiring decisions. (If they are missing any of those 3 factors, they may have input in the hiring decision, but I would not recommend a 1 on 1 interview or give them veto power).
Courtesy Interviewers: These people occasionally have something important to contribute, but should not significantly affect the hiring decision. These interviewers will probably be working closely with the new hires, but either have no detailed knowledge of the job, or are not seasoned interviewers with a proven hiring track record. While they may wish to exert influence on the hiring decision, I find they often add more noise than light to the hiring process. The more obnoxious people in this category will loudly share their (poorly formed) opinion about the hiring decision, and can often derail a productive conversation about candidate's actual ability to do the job. In the final evaluation, "I did not like his handshake" should not be considered equally with "He has an excellent track record managing projects like this."
The key to managing courtesy interviewers is to learn from their input, but to not become too distracted by their opinion. Their opinion should not be given equal weight to a veto-voter, unless they have objective facts to share. Yes, you probably need to "keep them happy" but you can control their effect simply by how you structure the interview sequence. By scheduling these less skilled interviewers into a panel interview, you can still involve them, but you are also respecting the candidate's time, and diminishing any adverse impact they may have on both the hiring decision and the candidate's perception of your company. To develop the interview skills of a courtesy interviewer, be sure to have them methodically record their interview feedback, put it in a drawer, and take it out and look at it 6 months or a year later.