I have done a *lot* of interviews these past few weeks. Enough to speak with some authority on the subject. Here are some tips.
1) Wear a shirt and slacks at the very least. A tie is good, a suit is better. Regardless of how casual the job you're going for is, you should make some effort to have a visual impact. It shows you're taking the interview seriously. It shows that you are behaving in a professional manner. Showing up in jeans and a tee shirt only gives the impression that you rolled out of bed and came in for the job and honestly, if you can't make that tiny amount of effort for an interview, why should we believe you'll make any more effort to do the job well?
To that end, I will slap the next person who thinks it is okay to come in in ripped trousers.
2) Arrive absolutely no more than ten minutes early. That way if your clock is off by five minutes you'll either be just on time or acceptably early. If you arrive too early then the interviewers will feel obliged to take you in for the interview before they're ready, they're flustered and pissed off because they probably haven't had time to prepare properly themselves. They won't get to ask you the questions they want to, you won't get a chance to answer the questions the way they want you to and you'll lose out. Arriving more than ten or fifteen minutes (and fifteen is the absolute upper threshold of tolerance here) is a no no. Never ever be late. If you are late, call ahead and let them know. If you are late and you are asked "Did you have any trouble finding the place?" then say "Yes" because that placates us and answering "No" leaves us wondering why the FUCK you left us waiting and created a situation where we're going to have to rush the interview to catch up. We have better things to do than wait for you without explanation.
3) After arriving no more than ten minutes early, wash your God Damned hands and dry them thoroughly. Keep some tissues on you to keep them clean and dry. Nothing makes an impression like a cold, damp handshake that ends with a film of your sweat on their hands and a good case of the grossout. If you have a cold or are otherwise unwell, call to cancel and explain the situation. If you come in with a cold then you're being a dick. You're going to give it to us, make us miss work, miss interviews, cause way more work for our co-workers and generally screw things up. You are not so important that we're willing to overlook that.
4) It does not matter how skilled you are at the job. It does not matter how much you think you know about it. Your experience does not speak for itself. We are not telepathic, you need to tell us.
5) Answer the question at hand, do not digress or otherwise go off on a tangent. Do not say "Umm", "Ah", "Err" or anything similar. If you need a second to think then simply pause. You will not look stupid for stopping to think for a second before speaking, quite the opposite. You will look like an idiot if you punctuate every second or third word with a vowelly grunt.
6) If you are doing a phone interview then get to a quiet place half an hour beforehand. Get yourself a cup of tea, a cigarette if thats your thing, get the computer running in case you need to look up some answers. Make sure your keyboard is nice and silent because we can hear you type and that makes you look like a cheating, clueless asshole. Get someone to call you before the interview to make absolutely sure you have a good reception. If they're calling from abroad then do some research and give them the number they have to dial from their office. Trying to figure out international dialling codes and which zero you're supposed to drop is a pain in the ass. Do not move from your seat until the interview is done. Do not, under any circumstances do the interview while walking down a street. Once again, if you can't make the time for something as simple as the interview, why should we believe you're going to devote your time to the job? If you absolutely must break this rule, explain why this is the case and apologise because it is hateful to try and speak to someone who can't hear you and have to repeat questions or ask them to repeat an answer over and over again.
7) Research the company and the specific role you are going for. When you're asked what your understanding of the job is you should damned well be able to tell the interviewer what it is. Misunderstanding the role or getting it entirely wrong is interview suicide. If you think you're suitable for something entirely different to the job then you've just applied for then you've applied for the wrong one and you're useless to the interviewer. If it is a summer contract then telling us you're available in september just pisses us off because you've wasted half an hour of our time. Do not apply for something on the offchance, or just to have a go, we'll know by the end of it and if your application says one thing and you say another then you're a God Damned liar and not to be trusted.
8) This will lead on into #9 but - Stick a picture of yourself on the CV. A good one mind you, not that one where you looked dark and mysterious at a convention or where you were a bit out of it with your mates - something of you looking amused but serious (preferably in a shirt, which will go some way towards tricking the interviewer's memory should you transgress against rule #1). Interviewers only have a memory span of about a week. By Monday morning, you will be a dim and distant memory, deliberately obscured by heavy drinking and while usually a decision will have been made by then this is not always the case. You have to ensure that your impression can last through a week and probably fourteen other candidates (most interviewers do no more than three per day)
9) Timing is a bastard but here are some rough guides.
Before lunch on a monday is a good and bad slot. The interviewers aren't in their swing yet and they're probably not that awake. It is easy to make a good or bad impression here. Be clear and concise. Don't waffle and just answer the questions you're asked. If you drag the interview out and keep talking then you're annoying us.
Between post Monday lunchtime and roughly Wednesday afternoon are all good times. The coffee has kicked in and we're not tired yet. Just follow the rules above and you'll be fine. Be bright, charismatic and outgoing.
Thursday can be dodgy, we're sick of interviews by this stage and have probably seen a dozen people. Our memory span only lasts roughly a week anyway so on the positive side, you're at least towards the memorable end of the week. On the negative side, we're sick of the sight of you people by now.
Friday is dodgy as well but for different reasons. The end of the week is in sight but your interview may have been pawned off by the original interviewer and onto someone else because the thought of listening to the same tired, trite responses for the fourteenth time in a week makes us die a little inside so you really have to pull out all the stops. There is no middle ground with fridays, you'll either be the person who came in and brightened our day or the cunt who we're not sure, but we could swear found our interview questions on the web somewhere. As the day progresses it becomes increasingly precarious to the point where the final one on a friday will determine the mood we go into the weekend with and you do not want to be the person who pissed an interviewer off on a friday evening.
In short, if your interview is on a friday morning then keep it brief because we're quite possibly hung over. In the afternoon and evening, inject some humour into it, try to make us laugh.
10) Be careful what you mention as your interests, no matter how relevant they may seem. I've interviewed people who informed me that they loved computer games, they were raiders in World of Warcraft, their guilds were first on the server or in europe or in the world to reach certain achievements which is well and good but what you're really saying is "I'm going to be spending my time in work looking up strategies. My evenings will be filled with gaming and I'll probably be up very late. Chances are I'll be knackered in the morning and do sod all for the first hour before the coffee kicks in and I can start researching strategies again. Oh and since I'll be playing games at home all evening and then going into work to play them again, I'll be tired of seeing them and my skills will not be what they should." He was an exception due to his extreme dedication to the game but you should examine your interests and look for ways they could be a problem for the job.
11) At the end of the day, interviewers are like those horrible fucking shrieking harpies in that "Sex and the City" show, you know how they're continually dumping guys for entirely trivial reasons? Well we're a lot like that, except we don't piss and bitch and moan about not being able to find anyone afterwards because we're not blonde retards written by fucking cretins. It does not take much to turn an interviewer against you, you need to be good for the job, a good fit for the office environment, good for the projects we need people for and not annoy us in the slightest because you'll be doing that every - single - day and we're going to end up murdering you.