What questions do immigration officials ask witnesses for a marriage visa in Thailand?

Seems OP got enough information for his question. Thank you all for your input.

I am married over 17 years and have a son both have western passports as well as Thai. The IO arrived to our new home and asked to speak to a neighbour. My wife called to our nearest neighbour he came and the IO asked him do you know this man he said no but he added i hear his voice all the time lol. So nothing to fear they just follow a process and tick the boxes.

They will ask basic questions like how did the witness know you, for how long, have they seen you and your wife living together, etc. nothing to worry about.

That is common, I had to do it as well

To apply for a conversion to a Non Immigrant O visa based on being married with a Thai National. Immigration can ask for a witness to explain and tell that you are actually married and living together, there is no standard book with allowed questions.

Sometimes the witness is asked to join on application of the visa, sometimes asked to join at application of Extension of Stay and sometimes neighbors are asked during the house visit when your application is Under consideration.

It is part of the requirement: In the case of spouse, the relationship must be de jure and de facto.