One way to do it is, when you're setting up an appointment, just click your
cursor in the time box, two boxes to the right of "Start time" and "End
time", and manually change the times to something other than the half-hour
interviews. For example you can change the start time to 12:40 instead of
12:30.
30 minutes is the smallest interval that will appear in the new event form.
However, if you set your time scale to 15 mibutes, then select the 15 minute
(or multiple of 15 minutes) segment you want to contain the event, clicking
New will open the new event with the selected interval.