Tam Dean Burn is recovering at home after he was rushed to hospital on Saturday afternoon by an ambulance crew.
The 60-year-old was making his way to Easter Road for the Hibs-Celtic Scottish Cup tie when his assailant leapt from a common close.
Friends who were with the River City, Outlander and Fortitude actor said they were heading to the Waverley Bar for a pre-match pint when the man suddenly appeared.
The incident happened after a tribute event to the Scottish poet Tom Leonard, who died last year. About 60 people – including Mr Burn, Liz Lochhead, Joy Hendry, Kevin Williamson and George Gunn – attended the event at the Scottish Poetry Library in Crichton Close in the Canongate.
The eyewitness said: “We had just left the Poetry Library a few moments earlier and were looking forward to a pint before the football when it all kicked offl.
“This guy just appeared out of a close. I turned round and he had Tam’s arm up his back. Folk were coming out of shops and stopping in the street.
“It was a scary scene and could have been so much worse although we didn’t realise how serious it was at first. It was only when Tam said he had been stabbed that we knew he had been hurt.
“Tam later said he’d never seen him before and had no clue who he was. He was stabbed in the neck. It could have been really, really serious.”
The witness said the friends who were with Mr Burn bundled him back up the street to the Poetry Library and staff locked the door while his attacker stood outside.
An ambulance crew arrived outside within minutes and were warned that the suspect was still at the scene by a member of the public.
Shortly afterwards, Police Scotland officers descended on the Royal Mile and the man was subdued and detained. It is believed a search later uncovered at least two weapons.
The witness said: “The ambulance crew had to be told that the guy was still there in case he was a danger to them.
“Tam was back in the Poet’s Library and the guy was outside showing no concern about being caught. This is in broad daylight on the Royal Mile. People were out of shops watching what was going on.”
He added: “We were outside and the guy was still there It was quite a scene. He didn’t seem bothered about witnesses. He was threatening us as well until the police arrived.
“We don’t think this was a random thing. It was something to do with Tam’s River City character, which is pretty frightening.”
Following the attack, friends of Mr Burn took to social media to ask if he was OK.
It later emerged that he had been treated at Edinburgh Royal Infirmary before being released home.
Mr Burn was not available for comment last night but posted a tweet saying: “Thanks for all the messages of support folks. I’m fine honest & when it says I’m resting that’s not the actors euphemism so going to take a break from here.”
Police said a man had been arrested in connection with the incident.
A spokesman said the 42-year-old was arrested outside Crichton Close at about 4pm and a 60-year-old man was taken to hospital.