1. Assuming that everyone at Acme Design Firm reviewed your resume and your portfolio before your interview.
I have watched this happen over and over again. Invariably, what happens is as follows:
Employer: We didn't have our creative director scheduled for this interview. But we decided to invite her along. Do you have a copy of your resume for her?
You: No... Could you print one off for her from the PDF I sent along?
Employer: [frustrated, but not showing it] Sure, that wouldn't be a problem. Also, our Wi-Fi network is down and the conference room that we're in doesn't have a computer. Could you bring out your laptop and show your portfolio from there?
You: Errr... My portfolio only works from my Web site.
In these kinds of situations, you can hear the punctured balloon of your professionalism starting to leak air. From here, you need to claw your way out.
2. Talking about what you did instead of the process of how you did it—and who helped you along the way.
Yes, the portfolio is the most important thing you'll present as a designer in your interview. But unless you're gunning for a design job where you're the solo-flight do-it-all type, how you describe your work in the context of your client and your collaborators will have a major impact on how your work style and personality are perceived by an employer.
Designers who can speak about their work in sophisticated terms—who are aware not only of the outcome of their design work, but can also describe the process thinking that carried them through a successful project—ascend a few notches in the eyes of their interviewers. If a designer can transcend the making and talk cogently about their thinking, they often exhibit other qualities, such as the ability to adapt their working processes to solve new and challenging projects. In highly complex design engagements with new technologies, there is no cookie-cutter approach, and this flexibility is key to survival.
Plus, I'm sure your interviewers want to hear how much your team loves working with you. "We want to hire you because you're an amazing designer who's going to do career-defining work, but that we're all going to hate to be around you after two months..." Not going to happen.
3. Thinking that, based on your Internet research, you understand what it's like to work at the company you're now interviewing with.
This is my general rule of thumb: If you're doing research on a company, assume that their Web site is at least 6 months out of date—even if it just went live! And no matter how thoroughly detailed a company's Web site is, always have questions for your interviewer that plumb the details of how they work: their corporate process, how the work is done individually and in groups, the details regarding your job role, and an idea of the long-term vision of your employer.
This last question is key, because if your employers don't know where they're headed in three to five years, you won't either—and it helps for an employer to have a very clear understanding of what they want to accomplish. Otherwise, it can be hard to align the employees with the company trajectory.
Along the way, you could also read blogs or tweets from current employees to get a sense of what life is like at their office—but I don't think you'll ever get the unvarnished truth from those people unless you ask them point-blank in an interview—and ask for complete honesty. If they aren't truthful in the interview, then you're being told something very important about their corporate culture by what they aren't saying...
4. Forgetting to ask what you should bring to the (next) interview.
You should always ask when going to an interview what they're looking to see. And if you have multiple rounds of interviews, when you've shown your book for the first time, you should always ask the interviewer if there are any specific areas you should focus on for the next round.
And just like any client project, there's what a potential employer tells you to bring and what you should bring if you really want the job.
For example: When working as a junior or mid-level designer, what's most critical to show in your interview is a well-considered portfolio and resume. (Fun leave-behinds are neat, but they only come in handy if you're trying to overcome a gap in your portfolio. I'd rather see more time investment in the work in their book.)
As you start to move into senior-level design thinking, the expectations shift. The designers who land big-time jobs will craft portfolio material and process work into a presentation that specifically addresses points of interest for the role they're targeting.
This sounds like a big time investment—and it is! But if you've thought very carefully about how you structure your portfolio in a modular fashion, you can quickly re-arrange and shape material before an interview without a major time cost. Besides, wouldn't you go to meet with a new client with a range of project samples that address the needs that they're seeking to fulfill with your services? Same thinking applies for your interview. There is no "one size fits all" portfolio when you start reaching for larger opportunities.
5. Being dishonest about what skills you have, versus which will need to be acquired on the job.
You need to be brutally honest with your potential employers about any skill gaps that may exist before you start your job. This issue only comes out when the designer shows up for work—and it often happens with designers looking to get their "foot in the door." In the process, however, they often break their big toe.
For example: If you're looking to be a print designer at a design firm and you have a solid portfolio of logos, that doesn't mean that you're an expert when it comes to print production. So be up front with your interviewer that you don't know how to read a press sheet or estimate a print project.
It's totally fine to say you don't know something. Hell, in this industry it's amazing to consider just how much we need to know in order to succeed. If you say that you're hungry for knowledge, that is considered a desirable trait for a new hire. It shows you're aware of just how much you don't know.
So... did I miss any major gaffes here?