Generally speaking, it was a characteristic of Venerable Acharn Mun when he talked of the higher levels of Dhamma, that his manner of speaking and behaviour displayed an intense concentration appropriate to each level of Dhamma. The effect of this on those who were not used to his behaviour was to make them apprehensive and afraid, thinking that he was scolding them and chastising them with his words.
But in fact it was because the power of Dhamma is a force which was able to cause his mannerisms and behaviour to display these characteristics outwardly. Then as soon as he had finished his Dhamma talk, his manner returned to normal immediately; almost as if nothing unusual had happened a short time before.
Sometimes he would even tell some amusing stories and laughter would break through his awesome manner. So nobody was ever able to penetrate and look into his character truly.