Talking with a friend they mentioned that before the start of a movie in a Thailand cinema, during the pre-movie advertisements a film was shown asking to stand to salute the king, but that the Thai audience of the movie didn't stand.
As a foreigner, what is the expected behaviour in such cases?
In practice, yes, it's mandatory. Also note that it's not "advertising", but the royal anthem of Thailand that's played before movies.
Thai lèse-majesté laws are so draconian (up to 15 years in jail) that it's not worth taking any risks here, and there's precedent that not standing up during the anthem could land you in jail for two years. While I'm not aware of any foreigners being prosecuted for this specifically, they can and have been charged and jailed for other acts.
As a Thai I stand up whenever National or Royal Anthem is played. It is to show respect and is required by laws. In recent times these laws are relaxed in enforcement but what is wrong with showing respect. Would you stand if you hear your own National Anthem? That’s what you should do to others National Anthem as well.
My latest experience half the cinema stands and the other half remain seated. I choose to stand.
