This needs to be pleasant, or at least neutral. The face should be relaxed instead of tense to convey that you are comfortable with your role in encouraging and supporting the conversation taking place.

Smiles are always welcome and help people to relax. When we are tense, however, our smiles can look like grimaces. Practice smiling in a mirror and get comfortable with offering more smiles. This shows that you like what you are doing and that you enjoy the process underway, as opposed to demonstrating that you see the conversation as a dreaded task.