The adverb 顿时 indicates that an action was sudden and happened very quickly. It is most often used with verbs that express feelings, mental activity, emergence, and disappearance.
For example: to get excited, to become tense, to be filled with life, to sound.
顿时 is used in written language.
The atmosphere suddenly became tense
qìfēn dùnshí jǐnzhāng le qǐlái
The adverb 顿时 can be placed either at the beginning of a sentence or in the middle of a sentence. It is placed before a predicate expressed by either a verb or an adjective.
A quiet mountain village suddenly filled with life