But this did not stop her springing to life to say how highly she rated her Medea co-stars, what it's like starring in the new Harry Potter film and how puzzling internet search engines are (she's mystified by them).
It's pretty gut-churning watching Fiona Shaw murder her sons on stage as Medea, the spurned wife wreaking revenge on her husband - but audiences love it.
The award-winning actress has won rave reviews for her role in the Greek tragedy, which is tangible evidence - if any were needed - that hell hath no fury like a woman scorned.
"Playing Medea is a masochistic pleasure - it's the pleasure someone jumping out of a plane gets - it's terrifying and it's horrifying," she told BBC News Online.
"In order for it to be any good - to justify your existence - it has to be as good and as new as you can make it.
"But this need to make a thing new is a killer and it makes one very tired."
The strain of the show was evident. Shaw was only three hours away from another gruelling performance, and was dying to curl up for some much-needed sleep.
But this did not stop her springing to life to say how highly she rated her Medea co-stars, what it's like starring in the new Harry Potter film and how puzzling internet search engines are (she's mystified by them).
Shaw reflected that her enthusiasm for acting and communicating were evident while she was growing up in Ireland.
"I was very good at doing poems aloud at school and I was excited by it," she said.
This, combined with coming from "a very loquacious family - always arguing and chatting" meant it was unlikely she would grow up to be a wallflower.
After studying philosophy at University College Cork, and a prize-winning stint at Rada, work in theatre, television and film was swift to follow.
But despite her acting pedigree, Shaw is not averse to dabbling in mainstream film efforts such as Super Mario Brothers and Three Men and a Little Lady.
Medea and Super Mario Brothers are at opposite ends of the spectrum - "one is good and one is bad" - but this didn't matter to Shaw.
"My memory is not of the film but of my life while I was there - for that alone the experience is tremendous," she said.
"Working in north Carolina with Dennis Hopper and John Leguizamo was just an amazingly interesting time."
She also had no problem with Three Men and a Little Lady, which is something children enjoy, as is Harry Potter and the Sorcerer's Stone, which is still being filmed.
"I'm very keen on children's work - the pleasure they get from films they really 'get' is great," she said.
Shaw deliberately avoided the publicity surrounding Harry Potter, resulting from the book's all-conquering success worldwide.
Unaware that Coke holds the film's marketing rights, the actress said the most sensible way of working was to "cut out the hype because it is of no creative help at all".
'Emotional detail'
Shaw, who plays a "Muggle" - Harry's nasty aunt Petunia - said the young stars were coping very well, and that Daniel Radcliffe, who plays Harry, was "wonderful".
"I had a ball doing Harry Potter," she said, heaping praise on director Christopher Columbus, whose "values seem to be bang on, with the attention to emotional detail very much in place".
"That is what will make the film, not the hype or the special effects," she added.
But although Shaw talked warmly of working in film, she said it is "easier" and less satisfying than the hard graft of theatre.
Play's rewards
"I'm always relieved when I'm in a film. You're asked to be as good as you can be on a day, and they'll only do 10, 20, 30 takes.
"But in a play, you have to try and reach that best of 30 takes every night.
"The rewards are that you own it. I prefer, if I have the health, the self-definition of doing a play."
Despite her successes, Shaw's still keen to quest after new experiences - she'd love to work with acclaimed US actress Meryl Streep, once she has recovered from Medea.
And before the interview ended, she managed to jot down several pointers on using the internet so she could master its search engines. It shouldn't take her too long.
Medea is running at the Queens Theatre, London, until 14 April.