When recruiting a student or post-doc to join my group, I trim down the list of applicants to a few names (3–5) by looking at CV's, research/teaching statements, existing publications, etc. Then, I will write to the mentors/professors/teachers/PhD advisors they listed in their list of references. I will also write to people who would be obvious choice as a reference, but aren't listed (fresh PhD who doesn't list his advisor, e.g.).
But what do I ask these people? My “standard” email would be something like:
A former PhD student in your research group, Dr XXX, is applying for a position in my group. I wonder if you can give me a frank and confidential appraisal of his abilities and working style. How efficient is he, how quickly does he get things done? Is he able to communicate clearly, both in writing and orally, at a level you would expect from an good researcher? Does he interact well with other scientists, and work in a team?
At least one senior professor whom I sent such an email replied with a rather negative tone, saying “Dr. XXX is a good scientist. I don't want to comment on your other questions.” So, I am wondering: am I asking something I shouldn't? Or not in the right manner? What should you ask of applicants' references?