Actually, if they're laid back, and this is an email, I would use no salutation whatsoever, assuming you don't have the name(s) of the people you talked to. "Interviewer" sounds jarring to me.

Would it be worth sending a paper card? Might bump you up a bit.

To answer the actual question, I'd use "Dear."

I am assuming that you do know the interviewer's name, and if that is the case, I would use "Dear" as the salutation.

I wrote my thank you emails and cards using just their first name. No "Dear" or "Hi". My logic was that I had already been introduced to this person so using the first name was comfortably informal, but I still carried over the practice of excluding a saluation from formal business letters (such as the cover letter I had initially sent this person). Seemed like a good compromise.

I like Dear. It is hard to tick people off with a bit of formality. Just make the tone truly warm and enthusiastic. I'd also suggest a quick call to say if you didn't already that you would certainly accept immediately if an offer is made to you.

Yeah, if you know the person's first name, I would go for 'Dear x'. Otherwise, you risk being seen as overly informal in a business setting and, no matter how laid-back the company seems, it's always better to err on the side of formality and professionalism (whatever that is) in such situation. It lets them know that you take them and the job seriously.

What bearwife said about making it clear you are available immediately - sometimes (although probably not often), internal candidates are preferred because they can be put to work straight away, rather than having to wait 2-4 weeks for notice periods. If they were really that keen on the internal person, they wouldn't have even bothered interviewing you, most likely.

I clearly did not understand your question.

Yeah, I know their names - snagged the cards too, just so I wouldn't miss one. There were two interviews - one with the team who would be my direct coworkers and one with the marketing director, so two thank yous to send out in a casually relaxed manner tomorrow at 10 am sharp. I managed to get hung up on Dear versus Hi. Yikes. Dear is better, y'all rock. I think paper cards would be a mistake because the marketing director made a point of telling me she was leaving on vacation this weekend and then her boss is leaving the next week so it would be two weeks before I heard anything back.

Snork. Salutation anxiety! Hi, I'm a mefite and I would be delighted to overthink this particular plate of, I believe it is legumes, but perhaps chickpeas? Would you like pinto or black beans with that?

I just had the same anxiety with my first email only thank you notes. I interviewed at a company that made a point to tell me that they preferred everything digital and try to avoid paper as much as possible. I still feel like I should be doing something more.
I hope you get your job. It sucks when employers have people come in and interview just to go through the motions when they have every intention of promoting from inside. Of course once they met you they were charmed and realized they must have you at any cost.

Good luck!