A day after an attacker was foiled outside Notre-Dame Cathedral in Paris, tension was high as police boarded the train at Valence station.
But the suspect in the train's toilet was just an actor rehearsing a part.
He was taken off the train for questioning and eventually released.
The drama unfolded on Wednesday afternoon as the high-speed TGV buzzed through the French countryside  en route from Marseille to Paris, France Bleu website reports (in French).
The suspicious speech emanating from the WC went on for some time. The man, an unnamed Parisian comic actor aged 35, could be heard speaking loudly in English and reportedly Dutch too. It is not known what play he was rehearsing for.
The conductor decided to act, alerting his bosses who contacted local police.
The TGV made an unscheduled stop at Valence and even the military were called in. This should come as no surprise, considering France is under a state of emergency and an express train in France was targeted in a terror attack less than two years ago.
"Bearing in mind the current state of terrorism and the national alert level, the conductor wanted to avoid any risk or doubt and had the person questioned," a spokesman for French rail company SNCF told AFP news agency.
The suspect was taken to a police station outside Valence for questioning but it soon became clear that he posed no danger.
He said he had been preparing for an audition and had chosen to practise the part in the toilet so as not to annoy his fellow passengers in the first-class carriage.
Although the unnamed actor missed his train, one French commentator suggested that at least he could say he had given a convincing performance.
When a train conductor heard the English words "gun" and "weapon" from a toilet on board a high-speed train travelling through southern France, he feared the worst.