1. Some people are horrible at interviewing. - There will be interviewers who struggle to look you in the eye, appear bored with whatever it is you are saying or don't smile at all. It's a reality that not everyone can be a great interviewer so get used to it and don't let it shake your confidence, personality and interest in the position.
2. Many interviewers don't anticipate thoughtful questions. - A lot of the expect someone to either not really be invested in the position or ask the basic questions like the timeline for hiring for the position or work hours. Many don't anticipate questions about why they are part of the team and company, their career journey to their current position, what the company culture is like or even something really thought provoking like, "What about my resume and/or cover letter intrigued you enough to reach out for an interview? What skills and/or qualities did you feel I possess that align with this position?". Be the one to stand out and ask questions!
3. Many times you don't get a response to your "thank you" emails. - While it is an unspoken rule to always send a follow-up thank you email to whoever you interviewed with, it isn't a rule for them to answer. I've sent over 100 and maybe have gotten ten responses. It is a very rare thing, but it doesn't mean someone didn't get it. It also doesn't mean you shouldn't send one! Not sending a thank you note, reminding the person/people you interviewed with who you are, why you want this position and thanking them for their time, is pretty much career suicide. Send the email!
4. Group interviews require more personality and more focus. - I've been interviewed by more than one person several times now and I find the blending of personalities takes a bit more work because now there's more than just you and one other person. Also, you have to ensure you address the parties equally and give them equal amount of eye contact. It isn't bad, but it can definitely be a bit more stressful than just interviewing one-on-one.
5. Don't schedule interviews too close together. - Made this mistake not too long ago of having an interview uptown in the 50s near 5th Avenue and then not too long after having an interview down near Penn Station. Bad logistics on my end considering the first interview ran long forcing me to be late to the next one. When in doubt, assume an interview will take two hours and schedule appropriately. It is better to be 20 or so minutes early than to be late.