Appearance:
Make sure that the clothing you wear that day fits the work you're gonna do. If you wear your most expensive Italian suit and you're getting interviewed for a job in the sewers that ain't good. In your case I would just dress up casual, t-shirt with normal pants and a vest is always good. Make sure you shower and heck, even spray a little more parfume than usual.

Behaviour:
Sit down when asked to be seated, not earlier. Don't be too stiff with your answers and don't be too loose. If they ask about your hobbies always tell them safe stuff. "I motocross in the weekends and play football every friday" is the answer not to give. Basicly, that means you get injured alot so you miss alot of class. Best answer is computing, gaming, even making animations. That fits the college and is overall safe, unless you own a Wii.

Portfolio:
Include everything you ever made that you feel is up-to-date with your current skills. Don't show them an animation you made 3 years ago that even you don't feel good about. Just show 'em what you like best, that's what I did when I got my current job.

Answers:
This is the hardest, they're gonna ask you difficult questions and you're supposed to give difficult answers. Luckily my current boss was loose and I could get away with:

Boss: "What do you suspect from us?"
Me: "Mainly loan."

That's okay, they laughed and I got hired. I don't think school heads are that affective by humor though. Well I don't have much to say about this, just answer what you feel is right.

Any questions, bring 'em on.

PS. I've had 1 school interview and 3 job interviews and I always got accepted.

Well, I just brought my laptop and put a large folder on the desktop after the name of the job/school. That way the people know you atleast prepared yourself, in the folder I just put all my stuff. That was enough for a job but I don't know what a school is suspecting. I guess if you'd just like to show images you print them out and put them in a nice plastic cover. If you like to show animated files bring a laptop.

first impressions are everything... i've always felt that a particularly important piece is a watch. A nice watch makes a statement about you.

But yeah in terms of a portfolio interview.. i think a wide variety of work in your portfolio is key. Even in a job interview i'll take in sketches and preliminary work. It shows that you are in tune with the fundamentals and that you are well grounded and not all fluff and glitter.

But yeah invest in a nice watch

When I did my Oxford interview last year, I was told that these guys live and breath their subjects. I imagine that would be similar here - the person interviewing you will have to have a certain passion for their subject which they will want to see reflected in you. Don't be too "OMG IT'S MY LIFE" but make sure that you communicate how interested you are. One way to do this is to maintain eye-contact and respond when they're speaking to you - nod or something like that to show that you're engaged - and when they do ask you a question

MAKE SURE TO THINK BEFORE YOU ANSWER

They won't think any worse of you if you take a few seconds to structure your thoughts before you answer their question. This is essential as it stops you from babbling rubbish and will hopefully make your point more insightful, thus creating an interesting conversation.

If they ask you a question which you don't know the answer to, use what knowledge you have to speculate the answer.

IE: "When was the ___ of ____ created?"

"I'm afraid I don't know the answer, however I do know that ___ was made in __ and of a similar style, so would the answer be ____?"

When you're sitting down, make sure to not slouch in the chair - I usually sit on the edge of the chair and have my hands in my lap - and keep open body language throughout the interview. Try not to move your hands in front of your face when you speak/think (it sounds obvious, but we all make mistakes when under pressure!) as it'll interupt the flow of conversation.

Try not to get too nervous! Everyone else having the interview will feel the same way as you and will probably all make mistakes! Try doing some practise interviews before with teachers/parents to prepare yourself for the experience.

Good luck mate!

1) Learn about the university and the course. A thorough grasp of knowledge about that will make you appear interested, which can set you apart from other candidates. It will also give you confidence.

2) Stand, smile, and shake hands when you meet him. If you're bringing a bunch of stuff in with you to the interview room, make sure you're hand is free so you can shake his... I've almost made that mistake before.

3) Think about asking pertinent questions. He may offer to take questions at the end of the interview. If you can think of any good questions, go for it. Ensure that your questions make you seem genuinely interested in getting admitted to the course, instead of being the self-serving type.

Dress up perhaps slightly above the way you would dress up for an average class day... just make sure the clothes you do wear are in good condition (well-pressed, clean, etc.) A cap may not be a good idea because it may make you look nonserious. And accessories, collogne, etc. should be somewhat understated.

Don't be late... 5-10 minutes early is the best time to arrive. You may need extra time for student parking as that can be a mess, so prepare for that. Arriving late is not good. Arriving too early just shows that you are nervous. If you must wait to enter his office, look around for flyers and handouts to learn more about the universty and/or class.

Use erect, but relaxed posture. Don't seem too overconfident. Give concise, tactful, and honest answers. Prepare to write down important information after the interview. Try to pronounce his name correctly; if his name looks complicated as hell, then you might be able to ask a receptionist how to pronounce it.

Thank him for the interview after it is over.

To supplement Moose's advice on posture, don't stick out your legs or anything either that would make you look nonserious. Leaning slightly forward can make you seem eager as well.