It really depends on when you travel and on what route.

If you give some details about your travel, what date and route you want to take, you will get advice whether it is wise to make reservations or not.

It depends, look on the website of the train company. Often they say if the train will be full or not. Also book for this trains via the train company and not via interrail.eu because it is cheaper. 

On long journeys I recommend to reserve, but also in Summer or Weekends. 

In Switzerland it is possible to reserve seat on IC trains, but nobody does it, except Tourists that do not know this. 

In the worst case you might be thrown out of the train if it is completely overcrowded and staff can’t guarantee for safety anymore. People without reservation have to leave train then. But this happens not so often… but it MIGHT happen.
 

The only 2 countries from where I´ve ever seen/heard this are DE=germany and AT=Austria. And then only for hi-speed sectors.

Now here in DE there is the ultra-cheap 9€ ticket for the whole month-for local (always UNRES anyway) trains and this leads indeed to gross Japan style overcrowding in some cases-then often it is told that BIKEs have to go out to make more room.

But I see usted Espanol: its no different from the MAD cercanias-in hi-peak and when a train (10 coaches duobledeck) has been cancelled and anyone tries to scramble into the next one=double load.