So, if you have a number of potential faculty who you could ask, then you may want to assess this dimension. The best way to do this is to talk it over confidentially with faculty who know you and who are committed to your future success. For example, in this case, you might seek out guidance from the pre-med advisor or from your faculty advisor. Alternatively, you might, in your letter requesting the reference, describe the kinds of things for which medical schools are looking. Now, for the more delicate issue of will they write YOU a good letter.
Faculty vary in their practices.
That is, if I feel that I do not know a student well or if I would prefer not to write for them, I tell them so. However, some faculty do not have this policy. I'd appreciate it if you would let me know how things work out. Feel free to forward these columns to anyone who you think might be interested. You are free to share these columns with your friends, your parents and your friends' parents, so long as it is exclusively for personal use.