The unwillingness to write a letter does not reflect particularly well on someone wishing to fill a position that requires such skills. It opens the door. Be on time. This may seem an obvious point, but it's an important and often misunderstood one. You see, while applicants are rarely late for an interview, they are often irritatingly early.
Most people looking to hire a new employee are busy.
Schedules are tight, and any bumps in the road can only serve to increase tension. If I'm not in a meeting and an applicant arrives 20 minutes early, I'm either obliged to start before I'm ready, or leave him or her waiting until the appointed time. Call me a softy, but that makes me feel a little guilty right from the start, and guilt generally transforms into resentment. A good rule of thumb is to arrive five minutes before your scheduled time, even if it means waiting outside for 15 minutes before going in. When I'm interviewing someone, I become our client.