Do I not need to supply passport details when booking through websites?

It depends where you're coming from and going to, however -- in my experience -- if the airline needs these details then either they will contact you directly, or they will get Expedia to do it. This often happens a little after booking, and sometimes they will wait until nearer your departure date.

If you're concerned the first thing to try is go to the 'Manage my flight' or equivalent on the relevant airlines webpage and try and access your booking via the information Expedia gave (hopefully you have the e-ticket number or booking reference, but often just name and flight are enough). These pages generally make it clear that you need to provide further info, and you can generally do it from that page.

If that doesn't work, and you don't get an e-mail, then contact Expedia support and ask them in the first instance. They should be able to help you.

We have travelled to Thailand for the past 8 years with Thai Airways and have always booked our tickets online; at no time have we ever been asked for passport numbers, just showing these at check-in. Never had any problems whatsoever.

Each time I flew Cathay Pacific to Hong Kong, purchasing the flight on Expedia, I was required to provide my passport information. However, I recently purchased a flight to the Philippines on Philippine Air and I was not required to give my passport information, nor does it appear from the expedia or Philippine airline site that it is possible for me to do so.

It depends mainly if your itinerary requires API (Advanced Passenger Information); and if so, you generally provide this at the airline's website - as they are ultimately responsible for forwarding this information to the relevant authorities.

API is required for most international travel (at least, mainly if you are traveling to Europe or the US) for some Asian countries its a hit and miss.

If you are going to be using any sort of express check-in service (like a baggage drop and online checkin) it is always better to provide this information.

If you plug in your reservation code and your last name at the airline website's "retrieve booking" function, you'll find out if you need to provide these details or not.

If you are a frequent flier with the airline, they generally keep your passport information linked with your FF number to make bookings a lot easier, so make sure once you have booked with Expedia that you update the reservation with the airline and add your FF numbers. For most airlines, doing so will automatically update your passport information on the reservation.