Never "over smile" otherwise the interviewers may have the impression that your smiles are fake or "put on".
Don't give stereo type of answers or else they may think that you have rehearsed the answers. Some people over the Internet who claimed to be "experts" or ex crew would have told you to rehearse and practise what to say.
When asked to introduce yourself,do not be long winded. Be precise and go to the point. Don't say anything that's not relevant to the job.
When asked why do you want to be a cabin crew, do not say things like " I love meeting people", " I like to travel the world", " I want to work with your airline which is the best in the world", " The salary is good" etc etc.
Do not stare at the ceiling or the floor when answering questions.
Do not mumble or speak too softly until the interviewers have to strain their ears to hear you.
Do not have too much eye contact with the interviewers to the extend of staring at them.
Do not frown or stick out your tongue when you give the wrong answer.
Do not speak Singlish.
Do not be too apologetic when you give the wrong answer.