Three years ago, an unknown actor called Patsy Ferran stepped on to the stage with one of the world’s best loved stars, Angela Lansbury. Ferran was playing the maid to Lansbury’s Madame Arcati in Noël Coward’s Blithe Spirit, and her performance put the 25-year-old actor on the map.

It transmitted much more than Coward wrote for the role — a jittery, nervous unsuitability for a life in service — and Ferran stole just about every scene in which she appeared. Since then, she has had increasingly eye-catching roles — from Portia in an RSC The Merchant of Venice to the androgynous Jim in the National Theatre’s Treasure Island. But this role fulfils the promise seen three years ago.

In Tennessee Williams’s rarely staged 1948 Mississippi-set play, Ferran plays Alma, the daughter of a minister. Since childhood, she has loved her neighbour, Johnny, the hell-raising son of the town’s doctor. He’s played by Matthew Needham who superbly embodies the spirit of a loose canon.

Rebecca Frecknall’s elegantly staged production is performed in front of a semi-circle of no less than nine upright pianos, by a barefoot cast, in front of the Almeida’s bare-brick wall and on an almost bare stage. Emotions are similarly exposed, even if in Alma’s case, they are initially suppressed. She is prim, Johnny is reckless.

They are compellingly ill-suited as lovers. She is painfully shy, he is cruelly confident and teases her mercilessly. Together, Needham and Ferran deliver a scintillating duet of opposites.

But Williams is concerned as much about belief systems as he is about character. Despite her religious background, Alma’s evangelical fervour is not so much in the service of God but in the intellect and, as we are constantly reminded, the true meaning of her Spanish name, the soul. Johnny, meanwhile is a cynical, bad-boy rationalist intent on wasting his talent. In one of the evening’s most potent scenes, he forces her in front of his father’s poster of the human anatomy and challenges Alma to locate the soul. So, much of the play’s conflict is about her high-mindedness versus his hedonism.

But the headline of this production is that Ferran elevates one of Williams’s lesser-known heroines into one of the great roles of 20th-century theatre, such is her command of conveying uncontrollable emotion. Her Alma is a hyperventilating bag of anxiety when we first encounter her. There are shades of that nervous energy we first saw in that Blithe Spirit cameo. But, here, it is the surface tension of a deep reservoir of intelligence and bravery.

That strength of character allows Alma to confront the object of her desire, question the painfully inadequate care with which she was raised, and reassess her life. There is no protection from the pain it causes, but her transition is among the most noble and moving I’ve seen on stage.

Thanks to Ferran, we will be seeing much more of Alma in productions to come.