Was there a strategic pause after these questions? When he said, "Can you imagine yourselves that ye hear the voice of God saying unto you in that day, come unto me ye blessed?" (Alma 5:16), did he just let that hang for a few seconds? Or, "Do you imagine to yourself that you can lie unto God in that day? And say Lord, our works have been works of righteousness upon the face of the earth?" (17). Did he then let that sink in just a little?
There are some very poignant moments of silence in scripture. For example, when the voice of God was heard from heaven and Christ descended at the temple at Bountiful, everyone awaited profoundly silent. Just imagine how that silence would be; nobody dared to say anything. They were frozen with anticipation and wonder at what had happened. Silence can be very powerful. Sometimes, conveying the spirit requires not talking as much and letting people ponder.
There are other techniques Alma used. For example, many of Alma’s testimony statements began with the word behold. Look at verse 13, "And behold, he preached the word unto your fathers and a mighty change was also wrought in their hearts and therefore they were saved." That is a clear statement of testimony. This is not hypothetical. When Alma said he was bearing down in testimony, he certainly was.
Jon D. Green, "The Paradox of Silence in the Arts and Religion," BYU Studies Quarterly 35, no. 3 (1995–1996): 94–131.