Most jobs interviews are meant to be a conversation, not a test. But no matter the style of the interviewer, there are keys to making the right impression.
The primary goal of an interview is to determine if you are the most qualified candidate for the job. But a secondary goal is to determine whether you are a good fit with corporate culture and the others already working in the group. The way you approach the interview indicates more about you than you might think.
I have interviewed hundreds of candidates at Fortune 500 companies and small start-ups. Based on my experience, I offer this advice.

1. Dress for success

Unless instructed otherwise, wear proper interviewing attire. Even if this means putting on a tie and coat on the way to the interview. Many companies with a casual dress code want to know you can present yourself well for the occasional customer meeting. Although, I recommend you don't go over the top (i.e., cuff links).

2. Be on time; don't get lost

Know the exact time and location of your interview; this might require a dry run drive to assess traffic. Traffic is not a good excuse for being late, so plan to arrive 10 minutes early. In case you do run late, have a contact number with you in the car so your interviewers can use their time wisely. On larger corporate campuses, make sure you know which entrance to the campus and which building you should enter.

3. Remember the interview is always "on"

Realize that the interview starts and ends right as you enter and exit the parking lot. You never know who might be watching from the windows as you: 1) exit your car and finish getting dressed, or 2) drop your papers and chase them around the parking lot, or 3) finish a heated argument on your cell phone.
The receptionist, administrative personnel, and anyone you meet are all a part of the team critiquing your soft skills. So treat other people you encounter at the company with courtesy and respect. Do not assume the interview turns on and off during your stay. Anything you do or say is part of the interview -- even small talk.

4. Watch the body language and tone

Offer a firm handshake, make eye contact, and have a friendly expression when you are greeted by your interviewer. Keep that eye contact and friendly demeanor throughout the interview. People want to hire candidates who seem like they would be easy to work with (and friendly). Smiling and politely laughing at the appropriate times convey more about you than you might think. The key is not to be stiff or nervous. Have a comfortable and conversational tone.
Also, sit still in your seat; avoid fidgeting and slouching. A simple way to avoid slouching is keep your lower back pressed against the seat. Doing this keeps you from leaning forward (appearing a little too intense) or slouching (and looking a little too comfortable). Keep your hands in your lap to avoid fidgeting; only hold on to your pen when taking notes. Candidates who click or tap their pen can be annoying during the interview.

5. Have concrete examples when answering questions

Your examples should be filled with details, but be concise in your wording. Remember, being concise ensures your intended message is not lost and allows you to bridge to related, important experiences. You should have a list of examples in front of you in case you get stuck.
When discussing your experiences, do not be afraid to use the word "I." Sure, everyone looks for a team player and you can say "we" whenever appropriate (e.g., "we brainstormed on the solutions, and I executed on the plan"). Ultimately the interviewer wants to know what YOU did versus the team.

6. Attitude is as important as knowledge

Although you might have options, treat each interview seriously and as though you are truly interested in the opportunity presented. You can always decide the job is not for you after you have had a chance to consider all your options and reflect on the decision. While in the interview, consider this job your BEST option.
Don't give the impression that you are only interested in an opportunity because of its geographic location. Or continuing education program, or company stability, or health club benefits, or salary, or ... you get the picture.
Remember, the interviewer is evaluating you as a potential co-worker. Positive attitude stretches beyond the workplace. They might even comment on the rainy weather to see how you respond. "Our lawns really need this water" is a POSITIVE response. Having a bad attitude includes making negative comments about previous employers or others.

7. Remember to interview them

Have intelligent questions prepared to ask the interviewer. Your questions should be ones that prompt a discussion about subject matter you are familiar with so you can be part it. You can ask procedural questions (such as "when will you make your decision?") on the walk out the door.
Your questions should help you determine whether you would like to work for that hiring manager and the company. Having no questions indicates you're not all that interested in the opportunity.

8. Listen as well as you talk

You need to be an excellent listener. Half of being an "excellent communicator" is being able to listen and understand what you are being told. Certainly ask for clarification if you do not understand a question, but if you have to do this too much, you will send a signal that you might not listen well (and therefore, not take direction well).

9. Capture the details

After the interview, make notes right away so you don't forget critical details. You can also jot a few notes during the interview, too, especially when getting answers to your questions. It sends a signal that you are listening and very interested in what they have to say. Just be careful about losing too much eye contact when putting too much detail in your notes. The details can be added later -- just write down enough for recalling the conversation.

10. Don't assume they read your resume

Yes, you read that correctly. Sometimes a company will throw a few staff members into an interview on the fly. Maybe just to get more opinions or to let you meet some of your potential co-workers. In these situations, they may know little to nothing about you. So expect to cover some of the ground usually addressed by a resume.
Another circumstance where this occurs is when an interviewer skims the resume and is more interested to hear what you have to say in person about your experiences. They key is to avoid thinking, "Gee, don't these guys do their homework? I'm concerned about how much they care about my interview today." Thoughts like these sometimes get telegraphed by your facial expressions.