Sometimes it´s possible but it´s not recommened as this can cause quite high penalty fares or even can be seen as fraud and you will get to know the Police at the next station :/  Atleast you have to pay a quite high purchase onboard fare. Bestway is to reserve lastminute via App (not the Interrailapp), at a ticket office or when possible at a ticket machine. It will be much less stress.

The trains are reservation compulsory to asure that everybody on the train have a seat (Different secruity restrictions as in other countries)

Most often these trains-€*, TGV in FR AVE in ES and some frecce in IT-have gates to enter the platform where they stop/start-so its simply impossible to do what you suggest. Railways are not that mad-and yes, other countries-other customs as in DB/DE-land, also much less delays.