Possibly due to poor preparation, possibly due to nerves, and possibly due to ignorance, people applying for and interviewing for faculty positions routinely make mistakes. Most of them are minor, some of them are major. Here are some I have seen (and a few I’ve done myself). Hopefully, by reading this you can avoid them.
1) Mentioning the spousal hire at any point before getting an offer in hand. Just don’t do this. You are interviewing for a job- focus on the job. You don’t want to bias anyone by making them think you will be a more complicated or difficult hire due to a spouse. You want them to evaluate you on your merits alone. Wait until you have an offer to mention the need for a spousal hire.
2) Aiming to be a +1 in your application materials. As mentioned before, the point of a faculty application is to get you an interview. You just need a decent CV, decent letter, and decent recommendations. You may be able to get a slight leg up on other applicants if you have amazing versions of any of these, but probably not. Most of the time if you aim to be a +1 you will fail and become a -1.
4) Fleeing your current position. No one wants to hire a jaded, bitter, and angry faculty member. You need to be chasing something great at the place you are applying for, not fleeing something terrible. You MAY say your current position isn’t a great fit, but you MAY NOT say it is terrible and you just want to be anywhere else.
4.5) Talking badly about colleagues. This is often seen in conjunction with fleeing your current position. I don’t care if your mortal enemy works where you work, you cannot talk badly about them. This is the image you are painting of who you are as a faculty member. If you talk badly about current colleagues, that means you will talk badly of future colleagues. You MAY say you don’t communicate well with a certain person, but you MAY NOT say they are a monster and make your life hell.
5) Giving a bad job talk. This is separate from phoning it in, but often occurs concurrently. You need to practice your presentation and make sure it is amazing. Most positions involve teaching, after all. If you can’t teach, you can’t do the job.
6) Being a boor. This covers a wide range of sins, including ordering numerous alcoholic drinks, not engaging people, being rude or dismissive, not smiling, not meeting people’s eyes, saying inappropriate things, and other unprofessional behavior. I’m not sure what to say to get you to not do this. Practice being a better person, I suppose?
7) Not having a clue. If you didn’t do your interview/site visit prep, or if you want a tenure-track position but are interviewing for a clinical-track position, or if you don’t know what the institution is about, you will turn people off. Do your prep work and make sure you actually want the job.
8) Being weird. Look, I am all for being outside of normal, but not during an interview. Dress conservatively, polish your conversation and interview skills, and don’t go off the rails in conversation topics. I once interviewed a faculty applicant who OVER-prepared and wanted to show it off (aiming for a +1) and, as a consequence, we didn’t get to talk about things that were actually important for the job.
I could probably go on. This isn’t intended to be an exhaustive list, but to give you a general sense of how to act during a faculty interview. You want to demonstrate that you will be a good, positive, productive colleague. No department chair wants a Project or a Problem Child. The more you can show that you get along with people and will do a good job, the better.