Monsieur
The summary answer is, it depends.
On...
- type of flight, scheduled, non-scheduled, private, commercial
- country of registration and membership of ICAO and/or economic bloc such as ECAC
- sometimes number of seats on aircraft (eg. Turkey)
- aircraft load eg. carriage of DG
- the countries involved and/or any tensions, embargos, restrictions
- cabotage (stealing internal business, eg. Russia and Turkey)
Scheduled airlines normally have bilateral agreements, but non-scheduled charter flights require permits - as above. So, for example BA123 LHR-DXB flies under bilateral agreement and traffic rights, whereas a charter airline operating the same route as a one-off would encounter more economic and national hurdles.