A new star turn – the actress upstaging Keira

It was billed as a play with two big names – Knightley and Elisabeth 'Mad Men' Moss. So how did Bryony Hannah steal the spotlight? Adam Sherwin finds out

They came to bask in Keira Knightley's return to the West End stage. But the show was well and truly stolen by an unknown actress, who worked as a barmaid to support her career.

Critics hailed Bryony Hannah as major new star after her "stunning" first night performance in a revival of Lillian Hellman's 1934 play The Children's Hour. Hannah, 26, was the unknown quantity in a starry cast including the former Oscar nominee Knightley, Elisabeth Moss, who plays Peggy in Mad Men and the Tony award-winning actress Ellen Burstyn.

Hannah plays Mary, the spiteful girl who gains revenge on two teachers at a New England boarding school by spreading a rumour that they are lesbians.

While Knightley was described as "competent" by one critic at the Comedy Theatre, there was no limit to the praise heaped upon Hannah.

She "acts the stars right off the stage" said The Times. Paul Taylor of The Independent praised a "truly astonishing performance". The "miraculous" Hannah, "in a performance of darting, furtive, fiendish malevolence, is a fantastic find," claimed the Daily Mail's Quentin Letts. The performances of Knightley and Hannah revealed "the difference between a celluloid mannequin and a top-flight stage actress", Letts said.

Hannah, who doesn't read the critics' notices, told The Independent. "The first night was so exciting. I've never known an audience reception like that. It was so warm, even though it was a press night."

Hannah was pulling pints in a Southampton pub when Knightley, now one of Hollywood's top-earning actresses, received her Oscar nomination for Pride and Prejudice in 2006.

The first congratulatory call she received yesterday morning was from her former employers at the Hampshire Bowman pub. "There were so many things I enjoyed about working there, even the smell of barrels of ale," she recalled. "It was a really special time of my life. My landlady Heather sent me flowers and called to asked me how it went."

Hannah's breakthrough could help more British actors gain plum roles in a West End which Sir Jonathan Miller accused of becoming "intoxicated with celebrities and stars".

There is already talk of The Children's Hour transferring to Broadway. "That would be the dream," said Hannah. The London cast would have to work around Moss's commitments to the Mad Men series.

The critics have described her character Mary as "demonic", a description Hannah finds hard to accept. "I don't see her as malevolent. That description surprises me. I really care about her. She's a girl who hates being called a liar and is fighting her corner. Mary is more than just a real bitch."

The daughter of a teacher and a retired Royal Navy lieutenant-commander, Hannah spent the money she earned behind the bar on trips to London to appear in fringe theatre productions. She won a Laurence Olivier Bursary in 2007 and was accepted into RADA. Hannah has since appeared in the National Theatre productions of War Horse and Earthquakes in London. She also had a small role in Ricky Gervais's film, Cemetery Junction.

The Children's Hour continues at the Comedy Theatre until 30 April.

Does Hannah want to follow her co-star Knightley into blockbuster Hollywood films such as Pirates of the Caribbean? "I really enjoy stage work. I don't think anyone would pay to watch me in something like that," said the modest actress. Those who have witnessed her West End triumph would no doubt beg to differ.

What the critics said

Paul Taylor, The Independent

"The one truly astonishing performance is that given by Bryony Hannah as the malicious pupil, Mary. Imagine that intimidatingly precocious brat who plays Katharine Hepburn's kid sister in The Philadelphia Story possessed by an unappeasable demonic hunger."

Libby Purves, The Times

"You think you've come to see stars. But you find that what you are actually watching, at first, is a diminutive, scowling unknown in a rumpled gymslip who acts the stars right off the stage."

Quentin Letts, Daily Mail

"[Knightley] does well in action scenes, when our eye keeps being drawn elsewhere – usually to the miraculous Miss Hannah. The outwardly plain Miss Hannah, in a performance of darting, furtive, fiendish malevolence, is a fantastic find. She has that native fizz, that crackle every time she enters. That's the difference between a celluloid mannequin and a top-flight stage actress."