When you are being interviewed it is very important that you give out the right signals. You should always look attentive - so do not slouch in your chair. Never lie to anyone in an interview, your body language and tone of voice or the words you use will probably give you away - classic body language giveaways include scratching your nose and not looking directly at the other person when you are speaking to them.
If you have a moustache you may want to consider shaving it off - people with moustaches can be perceived as being aggressive. You can always grow it again once you have got the job.