When Peter Hall directed a disappointing Othello at the National Theatre (with Paul Scofield, and Michael Bryant as Iago) you could see where his heart was: at Glyndebourne, where his production of Britten's A Midsummer Night's Dream was one of the greatest ever seen in the old country house barn before the new theatre was built.

Sir Peter, never let it be forgotten, is the outstanding figure in our post-war theatre, the architect of our unwisely and peevishly threatened subsidised theatre sector throughout the nation, founding genius of the RSC and battling hero in the transitional phase of the National between Olivier's Old Vic glory days and the modern era on the South Bank.

But he had his off days, probably because the strain of running the National was so great in the early years. And it was odd that Scofield, such a great actor, suffered a failure as the Moor of Venice much as he had done (also with Hall directing) as the murderous Macbeth at the RSC shortly before Hall handed over to a 27 year-old Trevor Nunn.

In so many ways, Nicholas Hytner has been the beneficiary of Hall's (and of Nunn's) career, but he still seems to have glided effortlessly through his NT regime and I very much doubt that we will learn of dark nights of the soul and bouts of depression as we did in the memoirs of both Hall and his successor, Richard Eyre. He seems to be made of steel, as stern and resilient as he is unflappable and inscrutable.

I think this shows in his work, which is often brilliant but sometimes cold. His own Othello, with Adrian Lester and Rory Kinnear, is excellent, but it doesn't tear you apart. This is a little unfair, perhaps, because no actor has really conquered the last act since Laurence Olivier did in his National performance 50 years ago.

Hytner's work in the theatre, though, is always on a par with his opera productions, and I spent most of yesterday watching a final dress rehearsal of his and designer Bob Crowley's beautiful Don Carlo, returning to the Royal Opera House with the current new superstar, Jonas Kaufmann in the title role.

Admittedly the German tenor was singing well within himself at the run-through, and his acting seemed a little, well, floppy, but  what a voice he has, a gift from the gods, and what a spell-binding performance this is certain to be at Saturday's opening. And Hytner's staging - which is clinical, geometric, lucidly expressive - contains all the emotional power it needs from the music alone, and the singing of Anja Harteros as Elizabeth, Mariusz Kwiecien as Posa and the indomitable Robert Lloyd as the old king.

There is a golden Spanish cathedral frontage, a frost-caked forest, a bonfire of heretics, a palace courtyard of red walls and cedar trees, a sharply illuminated throne room, an avenging mob of carefully drilled downtrodden peasants... it all looks absolutely magnificent, the work of a creative team (the revival director is Paul Higgins, the lighting by Mark Henderson, the movement by Scarlett Mackmin) at the peak of its powers and brimming with confidence.

Mind you, Hytner has a special affection for Schiller's original play, having directed a superb production in his time at the Royal Exchange in Manchester; the cast on that occasion included Michael Grandage, who later directed his own fine production at the Donmar Warehouse. But, boy, is it so much better with a band and Verdi's sumptuous music.

I sat goggle-eyed, and goggle-eared, at the back of the balcony alongside the director Irina Brown (soon to take charge of a Stravinsky and Tchaikovsky double-bill at the Bolshoi in Moscow) and several former singers in the chorus. We had the most wonderful afternoon... which then got even better when I cut along to the Harold Comedy to catch the opening night of Maria Friedman's brilliant revival of Merrily We Roll Along, newly transferred from the Menier Chocolate Factory.

Sondheim and George Furth are a very different proposition to Verdi and Schiller, but no less accomplished in their own ways. Irina told me at the opera that she was about to recreate a short Shostakovich piece for the South Bank that has lately been produced by the American director Peter Sellars.

This reminded me of the time someone asked Sellars at a Press conference if he thought that Sondheim wrote operas. "No," he replied, "Mozart writes operas." And so does Verdi. But Sondheim, like Mozart and Verdi, writes great musical theatre. And I think I now like Merrily almost as much as I like Company (the other Sondheim collaboration with George Furth); then again, do I like these two gems as much as I like A Little Night Music or Into the Woods...? And then there's Sweeney Todd. Oh, I give up.