If you were a program director (PD), you'd be trying to avoid two big headaches, as you assessed a residency candidate:
1) Will this person be competent and collegial? A PD does not want to get complaints from patients, faculty, or other services about his/her residents.
2) Will this person leave the program prematurely? A PD does not want to scurry around to fill an open call schedule or residency slot.
As you approach you interviews, consider how you can demonstrate your competence and collegiality (academic success, strong evaluations, extracurriculars that demonstrate teamwork) and commitment to the field and residency program (research projects in the specialty, knowledge about the program and city). Ensuring the PD knows you are not going to cause him/her headaches is half the battle.