When Richard E Grant sang the Swazi National Anthem on Australian television two years ago, little did he know it would land him his first musical theatre role in My Fair Lady.

Opera Australia director Stuart Maunder caught the Swaziland-born actor's appearance on Andrew Denton's Enough Rope in 2006, and immediately thought he would be perfect for the part of Professor Henry Higgins.

"I think the very first time I saw him was, thanks to my daughter, in (the Spice Girls movie) Spice World," Maunder laughed.

"But he did that Denton thing ... and I thought he was totally charming, totally disarming, and he did exactly what Higgins does when he wrong-foots everybody and he plays with everybody.

"I rang his agent ... and he immediately responded saying it was something he was interested in doing."

Grant, who has starred in more than 60 films, including Withnail and I and Gosford Park, said it wasn't quite an immediate yes - first he had to find out if he could actually sing.

"Straight away I got a singing teacher who specialises in getting actors who have not done a musical before to do it," he said.

"She said, 'I'll give you one session and if I think you can do it then say yes to the contract and I'll take you on'."

Grant's voice passed muster, and on Thursday night he will step into the role of the witty, arrogant linguist for the new season of My Fair Lady at Sydney's Theatre Royal.

Stage veteran Reg Livermore, 69, played Higgins for the Melbourne, Brisbane and Canberra seasons of the production, alongside Taryn Fiebig as Eliza Dolittle.

Fiebig said 51-year-old Grant brought a different, more youthful energy to the role.

"Richard has instilled it with so much vitality - I think (Eliza) has met her equal with Richard," Fiebig said.

"Some of the lines now have a different meaning because of the closeness in age. It's given it a different life."

"A sex life?" Grant interjected.

"Yes," replied a giggly Fiebig.

The show, which also stars Blue Heelers actor John Wood, was written by Alan Jay Lerner and Frederick Loewe based on George Bernard Shaw's play Pygmalion.

It made its Broadway debut in 1956, starring Rex Harrison and Julie Andrews, while a film version starring Audrey Hepburn was released in 1964.

A new film starring Keira Knightley and Daniel Day-Lewis is reportedly in the works.

Grant said My Fair Lady was such an enduring classic because of the strength of its story.

"It is a perfect musical," he said.

"It has a brilliant script that stands alone and tunes that have lasted 50 years and become classics in their own right."