The former Doctor Who star was praised for his portrayal of Hamlet – a part he had always wanted to play – with the Royal Shakespeare Company.
But afterwards he admitted being daunted by comparisons with great actors before him.
The 41-year-old Scot said performing in plays was “utterly terrifying”, and he felt so “scared” and “ill”, that it made him question why he had chosen to do it at all.
But he added that, like childbirth – despite the pain and terror involved – when it was over he had wanted to do it again.
Speaking on today’s BBC Radio Scotland’s Janice Forsyth Show, he said: “Hamlet certainly was something that I always had in the back of my head, that if ever that opportunity arose it was something that you had to take.
“There’s a line in Withnail and I – Richard Griffiths says it – ‘I awoke one morning and realised I would never play the Dane’, which of course is rather melodramatic and ludicrous, but there is part of you that as an actor wants that chance, to be part of that club.”