Your job during one-on-one interviews is to sell the candidate on joining your company without appearing desperate, and grilling them to see if they can handle the pressure.
You’re a salesman/interrogator. You want them to like you, but also to feel vulnerable.
This may sound harsh, but it’s very effective: my goal when interviewing someone is make feel like they won’t get the job, yet make them want it more than ever. This will pay off when it comes time to negotiating a salary when you offer them the job.
For information on this topic, check out: Leadership at 100MPH.