The most important thing to keep in mind, when interviewing rape victims, is the fact that they are a victim. Regardless of the circumstances which surround the rape, the person whom was assaulted did not deserve the attack.
Outside of that, one must also consider that the person is unsure of the process. They, most likely, have not been in this situation before and are fearful of what will come. They may be afraid to talk based upon these fears.
Another major factor that one must take into consideration is the fact that the victim may not wish to press charges.They may be far to worried about the court proceedings, the telling of their own personal history, and explaining why they were in the place they were where it happened.
The victim may also know their attacker and be afraid to accuse them. This could be far more harmful to them (in their own minds).
One last thing to keep in mind is the fact that they may have repressed the attack. Their mind, in order to protect them, may have pushed the facts of the attack far into the recesses of their mind. No matter how much one questions them, they may find it hard to recall the attack.