You can add train to the pass also one minute before travelling. 

I would suggest to look before if your destinations have some limits than to plan before, like Spain you need for almost all trains a reservation that is not possible to do online but only at train stations of RENFE Spain, DB Germany and maybe at NS in Netherlands if they can use the DB ticket system. 

Or France, Italy, Thalys, Eurostar, Sweden, Poland have high speed trains and long distance trains with compulsory reservation and TGV France to the rest of Europe quota for Interrail users. 

Nighttrains needs a reservation.

Germany, Austria, Switzerland, Czech Republic, Denmark,... have only optional reservations also on high speed trains and long distance trains. There is more easy to “plan” last minute. 

As angelo writes. In fact I´ve (after some 12+ passes) sometimes even only decided when I saw the screen with all dep. what next train would be-which once led to an interrogation by a booby in Doncaster station (I could hardly understand his brand of /en/) that this was apparently very unusual.

NS currently don't sell tickets/reservations for journeys to/in Spain. They will be adding Spanish destinations again, probably later this year, as soon as they're able to issue e-tickets/e-reservations for Spain.