"I laughed. He didn't. 'Ah', I said, cutting off the laugh at the pass and nodding wisely. 'A little bit of the Good Book before bedtime, eh?'
"'Yeah,' he said. 'Just a little at night. Keeps me on the right track, you know?'"
Armstrong was understandably surprised then when he discovered that Cruise was "cutting a wide swath through the local talent" in his hotel room after hours.
"Returning late one night, I found three or four young girls - late teens, I suspect - lined up in the hall outside of Tom's room," Armstrong writes.
"I remember thinking, 'Tom's going to be really upset if these hot girls interfere with his Bible reading. So I asked them, with all the stern gravitas of my 28 years, if there was something I could do to help them.
"They just stared at me, and at that moment, Tom's door opened and another girl came out, adjusting her hair and taking off down the hall, while the first girl in line slipped into Tom's room. This was a young man who knew something about time management and understood how to successfully juggle Bible study and blow jobs.
"I went to bed alone that night thinking it served me right for not being religious."
In Australia last month to promote his latest flick, The Mummy, Cruise revealed the secret behind his iconic dance scene in Risky Business.
Although it has become an infamous movie moment, the actor said he didn't realise at the time that it would be so memorable.
"I just ad libbed that," he said during an interview on The Project.
"I tried to go across [the floor] at one point and it was too sticky. What I did was I dusted the floor and then put stick [tape] on the other side so I would get centre frame on that and wore the socks. And that's how I finally did it - to figure out how to get that smooth, right on the beat kind of flow that got me there."
Director Cameron Crowe, who cast Cruise in Jerry Maguire, a role originally intended for Tom Hanks, said it was Cruise's voiceovers from Risky Business that helped get him the job.
"Tom had me at the first line, because the guy is so good at voiceovers," Crowe said in a Deadline interview in January.
"If you think about Risky Business, early on, he was wielding that weapon of being able to do voiceovers in such a warm, intimate, personal way. The first line of the script is like, 'so this is the world and there are how many people on it'.
"As he was saying that, I got a chill."
* Curtis Armstrong's memoir, Revenge of the Nerd, will be released next month through Thomas Dunne Books. You can read the full extract at The Hollywood Reporter.