A panel interview can be defined as when a single job candidate is interviewed by multiple people. This is different from a group interview, in which multiple job candidates are interviewed simultaneously. If you are being asked to participate in a panel interview, that is a good sign. It suggests that the hiring company is interested enough in bringing you on board to invest the time of multiple members of their staff - not a free proposition.
A panel interview is usually not like a military tribunal. It's unlikely that you will be seated in a chair in the middle of a large room with a menacing table of stern faces in front of you. More often, you will be seated in a boardroom or meeting room, and multiple people from the hiring company will be sitting across from or perpendicular to you.
In order to succeed in a panel interview, your actions don't need to be that different from in a one-on-one interview setting. You need to provide relatively concise answers that address the questions you are asked and that, when appropriate, include specific examples from your past and reference how your experience can be an asset to the firm.
Preparing for this interview isn't that different from preparing for any other type of interview. Research the company. Review your resume. Practice answers to critical interview questions. Dress the part, and make sure you know who your contact is when you arrive.
One pitfall in a panel interview is focusing your attention solely on whoever appears to be the most senior person in the room, especially if other interviewers seem deferential to that person. You will make the other people in the room feel uncomfortable if you do this. While the majority of your attention may go to this senior position, be sure to ask questions of, and sometimes address answers to, the other people in the room. However, don't turn into a lighthouse, with your eyes constantly scanning the room.
As with every interview, your body language and eye contact are important. Still, there is a bit of added pressure for you to express positive emotion in a group interview. Emotions are infectious. If you can win over one or two people with positive emotions, you are more likely to win over everyone in the group. Anxiety or negativity can similarly spread. So, in addition to your body language, be mindful of how you speak. Speak well of yourself and of others. Be confident and upbeat.
To build rapport with the interviewers, one valuable trick is to learn their names. To help you remember, repeat each person's name in three or more sentences: "Alex has a blue tie. Alex has a white shirt. Alex is sitting to my right." Then, when another interviewer asks you a question, you can say, "I think Alex mentioned previously that..." Alex now feels recognized and may become your ally. (If you know who your interviewers are ahead of time, do some homework to learn more about who these individuals are.)