Be prepared to go through a mock consultation with a potential client. Typically the interviewer will give a scenario; for example, "I'm a 45 year-old office worker with chronic lower back pain and the doctor told me I'm pre-diabetic", and then you need to be able to talk them through exercise prescription, nutritional recommendations, etc. Getting certified should have aptly prepared you for this.
Ultimately though, what you need to understand is that as a Personal Trainer, you're a salesman first, trainer second. Understand that you won't get to train anybody if you can't sell them on the idea of training with you. Salesmanship is key!
Learn to be a conversationalist, be charismatic, and inspire confidence in potential clients by being knowledgeable as well.