During an internal interview, the biggest, most common mistake people make is not saying enough and not putting forth enough effort.
Too many people go into this type of meeting feeling that they shouldn't or don't need to provide a lot of detail about their skills and experience because they feel the interviewer, who is usually a colleague, and maybe even a friend, already knows them.
You can't assume that just because you work with someone in one capacity, he or she knows all of the skills you can offer in another role.
In addition, some companies, in an attempt to minimize subjectivity, use points based interviews, which means they give points for specific things each interviewee mentions.
So, if the employer is looking for 5 points in your answer to a question, and you only provide 2 points because you assume the interviewer knows you know the other important points, you will not score well on the interview even though you may be the most qualified person for the job.
No matter how well you know the colleague who is conducting the internal interview, do not assume he or she knows all about your skills. Provide as much detail as possible when you answer questions.
The interviewer wants to hear about your relevant skills, experience and accomplishments. That's why you are being interviewed!
If the supervisor didn't need to hear candidates describe their strengths in an interview, he or she would simply choose someone for the internal posting and wouldn't waste time with interviews.
You need to assume that others who are also competing for the job will be highlighting all of their marketable skills and accomplishments. So, if you fail to promote yourself effectively in an internal job interview, you will not compare well against the competition.
Although the interviewer is your colleague, he or she may not be fully aware of all of your contributions to the company. The interviewer may have forgotten about some of your major accomplishments, or he or she may not know about some skills you have that you may not use in your current role but would be valuable in the new job.
Handle internal interviews in the same way you would handle any other job interview
Dress the way you would for a job interview. That means on the day of your interview, you should be dressed better than you would on a typical work day.
Bring copies of current, positive performance reviews, your resume and cover letter, and any other job search documents your employer has requested.
Do your research to determine what the supervisor is looking for in the person who will fill the role. Consider the supervisor's work preferences and the gaps that exist in the department. What does this supervisor value in his or her staff? Play up those strengths in an internal job interview.
Be tactful when you talk about improvements you've made in your department and improvements you could make if offered the new role. You have the benefit of inside information, so you should have an idea about where there's room for improvement; however, you must present those ideas without appearing to criticize colleagues who are currently involved in those projects. Get across the idea that you have and will continue to bring value to the company without being negative about your colleagues.
Ask specific questions about the new position. Questions that show you are interested in the job itself such as, "What is the most important thing you'd like to see me accomplish in the first three months if I'm offered this position?" show that you are truly interested in the job. Avoid asking self serving questions like, "Does this position come with a pay raise?" Questions like that tell the employer you are not really interested in the job itself and can make a negative impact on how the employer perceives your attitude towards work.
Finally, send a thank you note after an internal interview. It may seem odd to send a thank you note to your colleague, but it's completely appropriate, and it can easily help you stand out from your competition because most other people will not do it.