Suppose the applicant has to gather information about the company. The first and very obvious thing to do is to check out the agency's Web site. Being a part of a trade group where individuals of the same profession come together also will give the candidate more exposure to the field and maybe even to professionals who work in the same agency he/she's having an interview.
A less or mostly non prevalent way to gather info on your company is to find publicity events or articles which the company does. And then approach the PR professional of the company. This assures that you get firsthand information on the company.
Make it short and sweet
The most rampant mistake that candidates make is to keep talking and talking. Sometimes even without listening to the question properly! It's highly important to listen to the question properly. There are many candidates who don't listen properly and start replying. They most likely start talking and switch the direction of their answers mid-way unknowingly which doesn't answer the question the interviewers asked.
Give Instances
It is preferred that one gives examples of the experience they have and the work they did, the more the examples, the better. On our part, we should think in advance about the questions they might pose with regard to the post in question. The instances should be as fresh as possible and the answers should be to the point and precise, not vague. One should also enquire in the end if the said example answered their questions sometimes.
Honesty is the Best Policy
Many applicants lie about the skill sets they possess. If one doesn't have a certain skill one should plainly accept it. Beating around the bush and trying to avoid accepting a fact can score against you, if not in the present then definitely in the future. However, if one has skills which have even somewhat of a relation with the skill they ask for, you should tell them without hesitation.
Be careful
There are two types of interviewers. The first ones are rigid, serious and highly conventional. With them, the interviewees had better be prim and proper and take the job and the procedures earnestly.
The second types are the more dangerous types. They become the candidates' best bud. They make the candidate comfortable in their presence so that the interviewee opens up nicely. But many goof up here. They let down their guard down completely and tell them anything. One should always retain his/her sense of professionalism.
Do ask Informed Questions
One really good trick to impress the interviewers is to ask well read or well informed questions. Such questions show them that you have taken the pain to understand the company and the job well. Which also shows that you are really interested in the job.