Stone was touring Europe with two friends he grew up with in California. The three men in their 20s helped overpower a Kalashnikov-toting suspected Islamist militant on a high speed train heading for Paris from Amsterdam.
Among the other heroes of the night-time drama was a Frenchman on his way to the toilet who was first to tackle the assailant as he entered the carriage, and a 62-year-old Briton who still had blood spattered over his shirt as he spoke to journalists on Friday night.
But it may have been Stone, 23, of Lajes Air Base, Azores, who took the biggest risk.
"He was the first one to jump on him, he's the one who got cut up ... none of us are injured but Spencer took a few injuries and he just had no fear," 23-year-old student Anthony Sadler, told Reuters.
"That's our friend so once we saw him go we had to go and join him ... we couldn't have just left everybody die like that. It was a crazy situation, said Sadler, a student at Sacramento State University.
Sadler said everything happened very fast as the attacker, armed with an automatic pistol and a box cutter as well as the AK-47 assault rifle, appeared to try to clear his weapon which seemed to be jammed.
One passenger was hit by a bullet, and was in a serious but stable condition, authorities said. Stone is due to be released from hospital later on Saturday.
"I woke up to basically people ducking and then I was, like, 'Why is everybody ducking?' and then, when I turned round to look, he, the gunman, had just entered the car with the AK and then I was, like: 'This is really happening'," Sadler said.
"We just all ran back there and we tried to do whatever we could to try and beat him up so he didn't shoot anybody. He pulled out a box cutter and cut Spencer a couple of times but beside that we just tried to do whatever we could."
BACK FROM AFGHANISTAN
The third member of the group, 22-year-old National Guardsman Alek Skarlatos, said the vacation was partly to celebrate his return from a tour of duty in Afghanistan.
The men's fathers were thankful things turned out well.
"A guy comes back from Afghanistan and he has to fight a battle on vacation on a train in France," Emanuel Skarlatos said of his son, a college student who works at a Costco retail store as well as serving in the National Guard.
Speaking with Reuters by telephone from his home in Roseburg, Oregon, he said Skarlatos and Stone were as close as brothers, having grown up together in Carmichael, California.
"They trusted each other where one wouldn't back down if the other was getting his butt kicked," Skarlatos said.
"We're still trying to wrap our heads around it, but we're proud of him," Anthony Sadler's father, also named Anthony, told Reuters by phone. He said the three boys had known each other since middle school.
The elder Sadler, a 57-year-old pastor at Sacramento's Shiloh Baptist Church, said: "We're very, very thankful to God that he was not hurt or killed."
Asked if his son would continue his European trip, Sadler said "No, the trip is over... That's enough. He'll be returning home as soon as possible."
French Interior Minister Bernard Cazeneuve, President Francois Hollande and US President Barack Obama hailed the train passengers as heroes. As a show of gratitude, the head of SNCF French railways said his company would organise a visit to France for the Americans' families, should they want to come.
Cazeneuve said the attacker's identity was not confirmed, but that if he was telling the truth to his interrogators, he was a 26-year-old of Moroccan nationality identified as dangerous and with connections to Islamist militants.
Briton Chris Norman, who helped the Americans overpower the gunman, said: "Without Spencer we'd all be dead."
French movie actor Jean-Hugues Anglade, star of international hits Betty Blue and Nikita, who was also on the train, was quoted as saying by BFMTV: "We were stuck in the wrong place with the right people. It's miraculous."
