Enthusiasm in interviewing is, in most cases, a qualification red herring.
If one of the requirements is that hires show up enthusiastic every day, 99% of the candidates you meet will fail to do this.
Remember, you're not examining their ability to interview well or demonstrate spectacular emotion. You’re interviewing them to understand their ability to align with the needs of the role — and the role/company's ability to align well with their career needs.
Enthusiasm, or its close cousins "passion" and "charisma", are nice to have but in most cases not critical to shipping great work. Some of the best team members I've managed have been even-keeled folks that don't show a lot of emotion in either direction.
Interview for alignment, not emotion.
Interview for alignment, not emotion