How do you tell your boss “No.” without making it seem that you don’t want to be a “teamplayer”?

I have this side job that I really enjoy. It’s in the field that I have chosen to pursue and I’ve met some great people. However, the director of the program has approached me twice regarding duties that are unrelated to my job and that I feel are absolutely wrong for me.

He first approached me about a possible teaching position, and I explained that my recent decision to quit teaching had led to my applying for this position; as such, I am not looking to return to that field.

Tonight he approached me to see if I would act as a sort of student motivator, making calls to students who have stopped attending class. I tried to tactfully explain that I feel that I am not an adequate person for the job, as I do not like making phone calls and am not someone with that sort of sales-pitch personality. He insisted and advised that I give it a try and see how it goes.

I don’t want to give it a try.

I know this is not something I want to do. Partly, because I have a laissez-faire attitude to teaching – if you don’t care enough to show up, that’s not my problem. And partly because I have no desire to bother people while they are working/eating/relaxing/whatever.

I don’t want to seem like I don’t want to be part of the team, but this duty has nothing to do with my being a library assistant, and there are others who are better qualified to handle such a task.

I am hoping that it will not come up again, but if it does I want to be prepared to tell him no in a professional and well-argued manner.

I don’t want to lose a job that I actually enjoy, but I know that this is a task that will stress me out and make me feel as anxious as I feel at my other job.

Is it wrong that I do not want to do this? Should I accept a task that is unrelated to my job and most definitely not part of my job description? Do I offer to not work on the two nights that the other librarian is there, if it is a matter of my being superfluous?