The idea being, first allow the reference to ease by asking simpler questions, and then get into the meaty stuff. For this purpose, I like open ended questions, versus yes/no questions, that allow the reference to speak (and hopefully fumble) and not provide rehearsed answers.
What's your take on these questions?
1. How do you know (Candidate)?
2. How long did you both work together? Do you remember the approximate date (mm/yy) that you worked together?
3. What was the project about where the candidate worked?
4. What were (Candidate) primary responsibilities / role on the project?
5. What would you say were Candidate’s key strengths? Can you remember any particular area where the candidate truly excelled, versus delivering per expectations?
6. Now time for first important question.... Can you remember any particular area or instances, where the candidate's performance was below expectations? What were the circumstances? Did you offer any help to the candidate?
7. Where would you rank (Candidate) skills / compared to others on the team – Expert, Above Average, Average, Below Average?
8. How would you describe candidate’s ability to follow (or create) a structured project methodology?
9. Did the candidate provide any leadership?
10. How would you describe his/her personality?
11. No one is perfect, and everyone has some weaknesses. What were some of (Candidate) weaknesses? (See how answers align or deviate from question 6) (if I ask you what is 2+2 and again ask you what is 3+1, answer should be same)
12. How did he/she interact with the team members? With senior management?
13. Its time now to ask the REAL question - Why would you NOT hire the candidate? (This is a tricky one. Allow the reference a moment or two to absorb this question.)
14. How were (Candidate) work ethics – e.g. Coming to work on time, putting in enough hours, etc.
15. What else should I have asked you, that I didn’t?