Final answer:
The author is trying to convey that Usher's mood was constantly changing.
Step-by-step explanation:
The meaning the author is trying to convey in the sentence 'his action was alternately vivacious and sullen' is that Usher's mood was constantly changing. The word 'alternately' indicates a back and forth movement, while 'vivacious' and 'sullen' express contrasting emotions. This shows that Usher's actions and behavior were not consistent, but rather fluctuated between lively and gloomy.