From an employer's perspective, establishing and maintaining a rapport during the entire process is crucial. An employer should exude some happiness by meeting you and interviewing you, and the smiling is the best indication. However, if you get any single hint that the interviewer is not happy to meet you then clearly you haven't made a positive impression on them. Though it should be disheartening to go through such an interview, it shouldn't let you stop smiling at them. This will make you look interesting, pleasant, and more open minded about the whole interview process.