Bernard Shaw's ideal audience for this dazzling talkathon based on the Don Juan myth, written in the first years of the last century, was, he said, a pit of philosophers: so, thinking caps on and eyes down for three and a half hours of Ralph Fiennes speaking beautifully in the Lyttelton, staving off the the woman who loves him and the devil who taunts.

Simon Godwin's NT "previous" in the un-stageable category of drama with Eugene O'Neill's Strange Interlude has emboldened him to take drastic measures while including a good chunk of the "Don Juan in Hell" third act that is usually cut, though not when the NT first did the play in 1981 (with Daniel Massey and Penelope Wilton) and let it ramble on for well over four-and-half hours.

It's a cunning version, this, excising the difficult passages about eugenics and playing down the übermensch, Nietzschean superhero side of things, though Fiennes does resemble a sort of comical Shaw as John Tanner, bristling with an electrified energy from the moment he enters in modern dress. When Peter O'Toole played the role (the name is a near miss for Don Juan Tenorio, the Spanish original) in 1982, he and Lisa Harrow as Ann Whitefield (Dona Ana) played out a romantic comedy.

Fiennes and the delightful Indira Varma maintain that romantic strain but fill in the darker areas, too, while Tim McMullan as the chief bandit in the Sierra Nevada, and the devil in Hell, luxuriates in wickedness, offering sideswipes at the British on the mountainside and steaming cocktails in the underworld. Godwin makes it clear that the third act is a sort of dream and may even be suggesting that the entire play is, too, starting with a radio broadcast of Desert Island Discs in which a semi-comatose Fiennes is heard choosing Mozart's overture for starters.

It's a good idea to set the more fantastical Shaw plays in this sort of frame, or relief. The only trouble is that no-one else in the cast matches the Fiennes iciness or killer articulation. Nicholas Le Prevost is a lovely old Roebuck Ramsden (the Commendatore figure in the opera) but he don't half gobble and swallow his lines; he's too deft and light of touch for these tremendous paragraphs. Elliot Barnes-Worrell does better as the Cockney chauffeur Straker (the Leporello figure) and Ferdinand Kingsley has a charm and bounce about him as Octavius (ie, Ottavio) Robinson.

Octavius's sister, Violet, brightly played by Faye Castelow, carries some of Shaw's shock cargo in both having an illegitimate baby and marrying a rich American in secret. This strain, like some others, suffers from the cuts, so that the undermining of Tanner's carefully maintained pose as a free-thinking socialist revolutionary is less uproarious than it might have been. Still, Fiennes carries the burden of the play with enormous zest and skill, and it's a treat to hear him supervise a cascade of such brilliant writing (significantly, he played this role on radio in 1996, with Le Prevost as Octavius).

The designs of Christopher Oram are entertainingly eclectic, with a mountain range that looks like an adventure playground and a shiny wraparound Hell full of shadows, spooky lighting (by James Farncombe) and quirky musical allusions (sound by Christopher Shutt). The evening only becomes hard work after about three hours, and it's good for you. There was a time when we thought of Shaw as second only to Shakespeare in the canon and we might find his time coming round again in the centenary year of this great philosophical blockbuster's first full performance.

Man and Superman continues in the NT Lyttelton until 17 May. It will be broadcast via NT Live on 14 May.