The On Site Disney hotels offer the opportunity to meet characters within the Hotel. Times can vary, I have added them here and will try to keep this up to date but it is always worth checking for current times at the Hotel. 

The characters you can meet are themed to the individual hotel. 

The characters meet at a specific location within the hotel, they are not present in the hotel restaurants (with the exception of the Disneyland Hotel) and they do not wander around the hotel (with the possible exception of Disneyland Hotel, it's not clear yet exactly if sometimes characters may be in different areas).

Sometimes they will have a photo pass photographer but this is not always the case. 

Hotel New York, The Art of Marvel and when it reopens, the Disneyland Hotel, use a booking system through the Disneyland Paris App, which is only accessible to guests of that hotel once they have checked in (either online, up to 7 days in advance, or in person). With a reserved time there is not too much of a queue when you arrive. 

The other hotels use a regular queue and for Priority Pass holders they may:

• Not offer anything 
• Offer you to wait to one side and slot you in
• Offer a return time

This can vary between hotels and the cast members operating the meet that day.