Pause is not only one of the cues of prosodic structure for running speech, but also one of the strategies used by speakers to express their emotions. In this paper, we investigated the silent pauses in read and spontaneous speech, neutral and expressive speech, as well as synthesized speech, trying to find the difference of silent pause patterns in different speech styles. It was observed that all silent pauses in synthesized speech were regular, 2.5% pauses in neutral read speech were irregular, 10% in expressive read speech were irregular and 28% pauses in spontaneous expressive speech were irregular. A comparative study was carried out for the duration and frequency of occurrence of irregular silent pauses in different speech styles, as well as the valence and activation features of their acoustic contexts. A perceptual experiment showed that the removing of irregular silent pauses from spontaneous speech resulted in an obvious decrease of expressiveness, while inserting irregular silent pauses into synthesized speech improves substantially the expressiveness of such synthesized speech.