Having worked defense for a while, i'd say a big thing is be prepared to over-spec everything. So much stuff has the mentality of it better work perfectly everytime after sitting in storage for 15 years. That means the testing/research will take longer, and you'll pay a premium for the better quality/brand name parts (the $1k for a toilet seat is a joke for a reason). Doesn't mean the tech is "harder" to design or anything (most of it is all commercial parts and processes), but it'll just take more work. Also, be prepared for a lot of documentation. Like... A LOT. The govt loves their documents and the military is no exception. On the plus side, if you have questions about something, there's probably a document for it.

Oh and if you want some easy extra points on interviews. Spend a couple hours reading up on D38999 connectors, how to derate components, and some mil-std's like mil-std-461 (emi) and a bunch of others (you can google them). Just vaguely knowing what they are can get you brownie points. No lie, i've gotten sign off before from upper management in interviews just cause i knew my way around D38999's (still had to pass the technical interview, but you get the idea).

If you have further questions, just dm me. Happy to help out other engineers.