Preparation really helps in building confidence. Under-preparing can have disastrous consequences. It will make you look unprofessional, non-committed or disinterested in the job. Be punctual, introduce yourself clearly, make eye contact, be friendly and be yourself. First impressions count and are difficult to overturn.
Do background research in preparation. Find out about the director, company and as much as you can about the production. Consider other productions the director has undertaken and check whether they have a particular theatrical style.
Prepare yourself for any common questions such as Why do you think you're right for the job? How would you approach this play? Be prepared to talk about the play or the script if you have read it or to talk about the company's work with knowledge and enthusiasm. Don't be shy about asking any questions you may have. The interview is a two way process, providing an opportunity for you to find out more as well as for the interviewer to assess you.
Competition can be very high within the crew industry. Being considered for a film and tv job and then not getting it can be an inevitable part of the industry and something you'll have to get used to. Sometimes you might not have been right for that particular production or team. See it as good experience for the next occasion and see if you can turn rejection into something positive. Ask whether the interviewer would be prepared to give you feedback on the interview. Sometimes it is appropriate to send a message thanking the company for the interview and asking them if they would consider taking you on in the future. this can lead to an ongoing connection.
If you are called for an interview but are unable to turn up, make sure that you contact the agent / interviewer. Simply not turning up is wasting their time and cutting off any future contact between you and the employer.