Thanks so much for your reply! Yes, the countries I've picked seem particularly tricky with reservations but mostly Portugal and Spain. I'm assuming if I can't get a seat reservation I'll have to just buy a train ticket?
Spain, Italy and France are most cumbersome and their railways don't really seem to like Interrail, since their reservations can be expensive (especially for international trains from/to these countries) and the places where you can book seem to become fewer every year.
On top of that, especially Italian and Spanish railways don't send their complete timetables to the European Timetable Centre, so please don't use the rail planner app to plan, since it's incomplete (for Spain: very incomplete). Use their own websites (see links above).
Reservations for Spanish domestic trains can be made in Spain at the ticket office (it's usually sufficient to do that 1-2 days in advance) or via DB (German Railways, ticket office or by phone: +49 30 2970). You need to supply DB the train number (since their timetable will also be incomplete for Spain; look it up on the Spanish railways website). The reservation is on paper and needs to be sent via the post or collected at a DB ticket machine in Germany.
DB can also book international TGV/AVE trains from France to Spain, which is also on paper. You can also book them at a Spanish or French ticket office. Do book these trains this 1 week or more in advance, since the pass holder seats are limited, the number of trains too and they sell out well in advance.
French domestic reservations can be booked online via Interrail (2 EUR extra fee per person and train), via Belgian railways on their website (4 EUR extra fee per order, which can contain multiple people and trains, you need a pass cover number, request one here) or their phone number (no pass cover number needed, no extra fees but phone costs instead. reservation is also sent by email). Spanish railways cannot book French domestic reservations and probably no Italian reservations either.
Italian reservations can be book at an Italian ticket office (1 or a few days in advance is OK), maybe at a French ticket office and surely via Belgian railways by phone (will also be sent via email).
In Italy and France, there are often ways to avoid trains with mandatory reservations.
