During interviews, people are often offered a chair with a backseat. While it can be tempting for many people to sit back in order to relax, it is not a good idea.
2 The interviewer might think that you are not taking the job interview seriously or that you do not know how to show humility. Some people forget that they are in an interview when they are nervous, and they get inclined to sit back. Even if the interviewer asks you to relax, do not assume you can feel at home. You are there to get a job, not to feel comfortable.