Everyone has bad days, but no matter how pessimistic you are feeling on the day of your interview, it is important not to let it show. No company wants to hire someone with a bad attitude.
No matter how difficult your situation is, do not bring any baggage into the interview room. That means do not bad-mouth your former employer or any other companies you have been associated with and do not complain about your personal circumstances.
Be natural, expressing reasonable viewpoints through a lens of optimism. For example, if you have to talk about a challenging situation, you should mention how you may have helped solve it, and what you learned that made you a better employee.
Remember, your body language does matter as much as your words and is noticed by employers. Walk-in with a smile on your face, offer a firm handshake, and sit up tall at the table, leaning slightly forward to engage in the conversation.
Showing a positive, friendly attitude goes a long way in impressing an employer. Employers want to hire people that appear upbeat and are likely to get along with their bosses, co-workers, and clients.