Thanks for your interesting info on this production, tintin1056. Are you planning to go and see it?
tintin1056:
Anyhow, I still wonder if somebody wants to put a Shakespearean play on stage, or write a commentary about Shakespeare's plays somewhere in the world, does he or she require to obtain licence from Shakespeare's descendants? I think, in the world of art and literature, the rules are a bit different from the world of commerce.
To clarify your general thoughts on copyright, the reason you don't need permission from Shakespeare's descendants is basically because he's been dead so long.
The current legal position in the UK (and I think maybe in the EU generally) is that a literary- or artistic-work remains in copyright for seventy years after its creator's death, during which time you would indeed need permission from the late creator's estate to adapt their works.
Assuming that Belgium has a similar copyright law to the UK, and assuming that remains the case for a few decades, Herge's works will come out of copyright and into the Public Domain in 2053.
According to Wikipedia, Iran does have a copyright law giving protection to works for 50 years after a creator's death, but as Iran isn't signed up to the relevant international treaties, this only applies to Iranian creators and not to works from outside Iran.
So I think you're quite correct that as long as this production is only being performed inside Iran, it wouldn't need permission.