Lots of candidates leave it to the interviewer to determine the energy levels, tone and mood of the call. That is a bad idea. In our experience, the best candidates always bring a light mood to interviews. Seriousness, can hurt you as it is confused for anxiousness. Delegating the mood to the interview is bad idea since it means the energy of the case will be largely out of your control. Moreover, unless you practice controlling the energy, it is unlikely you will have this skill to deploy in the case interview itself.