The man told a jury he found Talbot, then a teacher at his school, stroking his hand in a sexual way during a weekend field trip to the south of Scotland in the early 1980s.
The witness said he felt “pretty embarrassed” and “naive” after the incident, and did not report it at the time because it would have been “my word against his”.
Talbot, 67, is on trial at Lanark Sheriff Court, where he denies indecently assaulting several teenage boys on school trips to Scotland.
He is further accused of lewd, indecent and libidinous practices and behaviour towards a boy aged 12 on a trip, a charge he also denies.
The witness, 51, who now lives in Australia, gave evidence from the other side of the world on the third day of the trial via a video link-up.
He said Talbot was a biology teacher at the school in the Manchester area where he was a pupil and recounted the time he went on a camping trip led by Talbot to the St Mary’s Loch area in the Scottish Borders in the 1980s.
The man, who cannot be named for legal reasons, told how the group put up their tents in wet and windy conditions in the dark and then went to a pub.
The witness, who was 15 at the time, said he had three or four pints of shandy and beer, and became “merry”.
When they got back to the camping area, he went to his two-man tent, in which Talbot and another pupil were also due to be spending the night, the court heard.
“I think I had been asleep and then I was aware that my hand was being stroked in the tent,” the man told the jury.
“The only person who could be doing that was Mr Talbot next to me.”
At first he did not realise what was happening, he said, but then became aware it was a deliberate act and not an accident.
“I can recall (doing) something to the effect of pulling my hand away and rolling over and saying ‘I don’t understand you’,” he said.
He told depute fiscal Imran Bashir the stroking was in a “sexual way” and added: “My feeling at the time was it was a sexual advance.”
The man said he told his friend about what happened, who laughed and said “he must have thought he was in with a chance with one of us”, the court heard.
The witness said he felt “pretty embarrassed” about what had happened, adding: “I think I felt I’d been very naive about what other people had said about him, that there was actually some truth in it, it wasn’t just rumour.”
Talbot’s media career later developed and he left the school, jurors heard.
“I remember thinking it was a good thing that he’d taken himself out of the place where he was tempted into doing things that were inappropriate,” the man said.
Alan Gravelle, defending, asked whether school gossip about Talbot had influenced his perception of what happened.
“I would say ‘no’,” the witness replied.
Mr Gravelle also suggested any contact may have been inadvertent and accidental.
“There’s nothing in my mind that would concur with that,” the man added.