As you say "not normally" but countries may decide to reinstate checks at borders, usually arguing a terrorist threat. Checks on cross-border roads are more frequent with the real reason being illegal immigration.

And it is mandatory to have an ID card or passport to fly around Europe, even on domestic flights (for security reasons). It will have to be shown at the boarding gate. A driving license may not be even accepted.

Thanks all!
 

Thank you to everyone who took the time to respond! I booked a return flight. Going to save myself the anxiety and possible headache/hassle. :)