Dear John, I always thought you were immortal. Ten years ago, when you were well over 80, we were in Tuscany and you were playing the disreputable, lecherous old journalist who insists on going on a summer holiday in order to embarrass his family, in a filmed version of Summer's Lease. You went home, had a small operation and came back a week later still chain-smoking, gossiping, talking endlessly, acting with a faultless touch, as though there had been no interruption.

I remember a long night shoot with you talking in a dark garden. We got on to your great comic creation, John Worthing and your memory of Bosie. "Lord Alfred Douglas was a beautiful young man who ended up sour and ugly. Can you believe this? He was at the first 'The Importance' and he couldn't remember whether it was played as farce or comedy. He was Wilde's closest friend and he couldn't remember a single thing about the production."

I suppose I was not much more than 10 when I saw your first Hamlet, an intelligent, witty, sensitive and poetic prince, for me the Hamlet of all time. "I played other characters," you said, "but I always thought of Hamlet as me." I can still see you as Richard of Bordeaux, a spoiled king, bathed in a golden light. I wrote for a signed photograph of you wearing a trilby hat and the star of the Good Companions. I never dreamed that I'd grow up to sit with you in the canteen of Thames Television and have you tell me that you learned more from your failures than your successes. When you were at the Old Vic, playing all the great roles in your youth, James Agate came round to congratulate you in the interval on Macbeth because, he said, "I might have changed my mind by the end of the play". As you talked your hands moved to the rhythm of your speech. "Ken Tynan said I only had two gestures, the left hand up, the right hand up - what does he want me to do? Bring out my prick?"

The pre-war theatre was dominated by two great stars. You and Olivier. You were the master of the poetry, easily moving the audience because you were so effortlessly in tears yourself. You told me that your mother cried "almost constantly like a wet April". Olivier, with the clipped delivery, was the physical actor, dropping from a great height to kill Claudius and rolling down a long staircase as Coriolanus. The rivalry became a collaboration when you and Olivier alternated as Romeo and Mercutio in an unforgettable production.

Shakespeare hadn't been played in the West End since the days of Irving and Tree, but you made him a box office hit on Shaftesbury Avenue and then, in the 50s, the theatre changed, Look Back In Anger came to the Court and poetry seemed to be no longer wanted. Luckily, it was Tony Richardson, also from the Court, who cast you as Lord Raglan in the Charge of the Light Brigade and a great player of film comedy was revealed.

You said that you had three besetting sins, on and off the stage, impetuosity, self-consciousness and "a lack of interest in anything not immediately connected with myself or the theatre". This lack of interest extended to horses. You played the King of France on horseback during the filming of Becket. The director told you to say the line and the horse would move one pace forward. At the first take you said the line, of course impeccably, but the horse remained immobile. By the seventh take the horse had still not moved on cue, or at all. Then, extremely puzzled, you asked the director: "Do you think the animal knows?"

It was true that you took a somewhat vague view of matters not connected with yourself or the theatre. One night in wartime you were seen to look sadly up at the barrage balloons which protected London and said: "I do feel so sorry for our poor boys up there. They must be terribly lonely." I remember a dinner long ago at Tony Richardson's house. My actress daughter Emily was a baby then, and we had brought her with us in a carrycot and left her in a spare bedroom. We were lugging this pink plastic box out of the front door when you saw us and said: "Why on earth didn't you leave your child at home? Are you afraid of burglars?"

You were also, of course, the most famous brick dropper of all time. Having directed Richard Burton in Hamlet, you went into his dressing room meaning to say, "We'll go to dinner when you are ready" but your critical subconscious made you come out with, "We'll go to dinner when you're better". Your classic dropped brick came when you were having lunch in the old Ivy with a pre-war playwright whose name, curiously enough, was Edward Knoblock.

A man came in at the door and waved to you and Knoblock asked you who he was. "That," you said with great confidence, "is the second most boring man in London." "So who's the first most boring man in London?" the playwright asked, to which you unhesitatingly replied, "Edward Knoblock, of course". And then, realising you had said exactly the right thing, you tried to mend matters by adding, "Not you, of course, I mean the other Edward Knoblock".

Apart from Summer's Lease you did a radio play of mine and you were unforgettable as Charles Ryder's father in a television adaptation I did of Brideshead Revisited. The scene in which, with barely concealed malice, you mock your son's unwelcome need of money must rank among the great comic performances of all time. You had that essential quality of all comedy performers. The scene must always be played absolutely seriously, and the actor must never think he is being funny.

So I have been extraordinarily lucky. You introduced me to Shakespeare on the stage, and you said lines I have written. More than that you were, as everyone who knew you or worked with you can testify, an inspiration to all of us and a constant, unexpected, unpredictable joy to be with. I suppose we must accept the idea that you are not immortal, but irreplaceable you most certainly are.