When Eamonn Andrews steps up to someone and says: This Is Your Life, it's the most tense moment of the programme. Or is it? The series is back this week and here Robert Tyrrell, who produced many of the most popular lives of the past two years, tells of even tenser moments... the stories behind the stories that appeared on screen.

There are two questions I am always asked about This Is Your Life: do the subjects ever know what is coming, and does anything ever go wrong behind the scenes?

The answer to the first question is (I believe) no, and to the second question – yes, sometimes.

This is the fascination of the show for those who work on it: you can never be sure quite what will happen. Surprise is the essence of This is Your Life, and sometimes the surprise is on us. We do our homework, we make our plans down to what we think is the last detail, but there are some things you cannot anticipate.

Vidal Sassoon's wife, Beverley Adams, joined in our plans to surprise her own husband. She was to deliver him at a precise time to the sixth floor of the Café Royal. We even made allowance for everyone to visit the loo before they left (loos can figure largely in schemes of this kind).

What we didn't think of was that Beverley herself would lose an ear-ring down the loo, and have to spend several minutes recovering it.

And that, Eamonn, is why you were left anxiously waiting by that lift.

Another occasion when the loo nearly foiled our plans was on the Henry Cooper Life. We planned to surprise him as he entered the studio itself, as the show began. Eamonn was to introduce the programme "live" on the air before the studio audience, then wait for Henry to come through the studio doors.

Henry thought he was to be interviewed about his then forthcoming fight with Jack Bodell for the Empire heavyweight title. He arrived at the studios, was ushered into a dressing room with his manager, Jim ("The Bishop") Wicks, and kept there chatting for a few minutes until 7pm.

Unfortunately, because of the geography of the building, Eamonn had to pass Henry's dressing room to get into the studio. We made sure that someone would shut the door just before Eamonn came by. We had also arranged for someone to suggest to Henry that he might like to go to the nearby men's room a little before seven.

It all seemed foolproof. But what we had not foreseen was that, on emerging from the men's room, Henry would hang about in the corridor outside his dressing-room, idly looking at the notice board on the wall – with Eamonn due to pass at any second.

For one wild moment, I considered pushing the British and Empire heavyweight champion into his dressing-room – then Jim Wicks came to the rescue by calling Henry in. The door was discreetly pushed shut, and Eamonn was able to slip past and into the studio. One minute later, Henry was led to the studio doors, still unsuspecting...

This "cloak and dagger" secrecy right up to the last moment is more, much more than just a gimmick. Every edition of This is Your Life is a surprise party, with all the excitement that normally comes only at Christmas, birthdays and other family reunions. If our guest of honour had too long to think about who might be coming through those doors, then half the fun and sparkle of the show would evaporate.

For us on the This is Your Life team, keeping the secret is second nature. We live in a world of buttoned lips and secret codes. But we have to rely on others for the toughest task of all – keeping a secret from a husband, a wife, a close friend. Joe Brown's wife kept the secret half-way round the world while she was on tour with Joe. Edward Woodward's wife lived with it for eight months while we waited for the best time to do his show.

Sometimes, a secret seems impossible to keep. Ronnie Corbett was surprised in the middle of a sketch he was doing on a Sunday night for a David Frost show at London Weekend Television. In it, he played the part of a man who believed that Eamonn Andrews was hiding in the cupboard waiting to capture him. At one stage, he even suspected his "wife" of being Eamonn in disguise. The pay-off to the sketch was to be the arrival of Eamonn himself, and we planned to cap that by telling Ronnie that it really was his life – to be recorded there and then immediately after the Frost show.

David Frost and London Weekend co-operated up to the hilt. Everyone was sworn to secrecy, but it was still a frightening situation. News travels fast in television studios, and there were dozens of people who had to know. While the Frost show was going on before a big studio audience, stage hands were setting the scene behind the backdrop for This is Your Life, and all Ronnie's friends and relatives were sneaking in through the back door.

It would have been the easiest thing in the world for a cameraman, the floor manager, a scene hand, anyone to let slip a clue that might have given us away – but no one did. The secret was kept until the moment Eamonn showed an incredulous Ronnie his name on the book.

What was our biggest back-stage problem? I think it was on Jimmy Savile's 'Life' last year.

We had planned an expensive hook-up with Leeds General Infirmary, where Jimmy goes every Wednesday to work as a hospital porter. The hospital was co-operating, technical surveys had been made, everything was set for a big surprise as he came all unwittingly through the doors of the children's ward. But then the bombshell hit us.

Only a few days before the show, the chief porter rang to say that Jimmy had decided to take a week off – at Torquay.

Somehow, we had to find another appointment that he would keep, where and when we wanted it, and Jimmy is not the most predictable or gullible of men. Researcher David Carter came up with an idea. Jimmy, he said, was an honorary member of the Royal Marine Commandos. He was proud of having gone through the tough assault and endurance courses at their training school in Devon. If they invited him to visit them, he would come.

A telephone call to the Marines (yes, we told it to them) found them ready to help. They would lay on a parade of Marines in Arctic gear, and ask Jimmy to inspect them. Operation Leeds was rapidly unscrambled, and Operation Devon was begun. Only 48 hours before the show, a signal came that Jimmy had accepted the invitation.

Eamonn was smuggled through Exeter station and into the camp. At the appointed hour, Jimmy arrived on the parade ground, and began to inspect the troops, drawn up in their fur-lined parkas. And half-way along the line was one man a bit taller and even burlier than the rest - with a book under his arm.

"When this fellow stepped forward and spoke to me," said Jimmy afterwards, "I thought he'd gone mad. I reckoned he'd be in the brig for 50 years for breaking ranks on parade. And then I saw it was Eamonn. But, even when he produced the book, I still couldn't believe it was for This is Your Life – and for me."

Wendy Craig gave Eamonn and the team a nervous time when she was captured last year. We had arranged with her husband, show-business journalist Jack Bentley, that the whole family should have their photograph taken together at the studio of a well known London photographer.

The plan was that they should be posed under the lights by the photographer, ostensibly for his camera, but in reality for the television camera we had concealed in the studio.

Wendy would then be called away to answer a telephone call from her agent, so that Eamonn could take the photographer's place under the cloth of the old-fashioned camera. On her return, he would emerge with the book.

Everything went smoothly at first. Wendy went to take the call, and Eamonn hid under the cloth. But when Wendy returned she suddenly decided that her hair needed attention, and went into the little dressing-room just off the studio, where one of the technicians was hiding.

With great presence of mind, he nodded, smiled and began to comb his hair in the mirror. Wendy smiled back and combed hers, wondering who he was and what he was doing there. Outside, bent beneath the black cloth, Eamonn peered through the camera viewfinder and wondered what had gone wrong.

"It was probably only a minute or so," he said afterwards, "but it felt more like an hour."

Eventually Wendy emerged, took up her pose – and the surprise was sprung.

Perhaps the toughest assignment a Life researcher ever had was to research the story of Richard Evans, the former lifeboat coxswain from Moelfre in Anglesey. For six weeks, Mary McAnally, a young English girl, slipped in and out of this tiny Welsh-speaking village, holding secret meetings with the family and friends of Coxswain Evans. To have managed this in an English village without starting a word of rumour would have been a remarkable feat. To anyone who knows a Welsh village, it is fantastic.

The final irony was Richard Evans's confession to me after the programme. He had always thought This is Your Life was a "put-up job", and that the subject knew in advance.

"Now I know the truth," he smiled, "I didn't have an inkling. Do you know, I didn't even recognise Eamonn Andrews at first – and I watch the programme every week."

Surprise – that's the vital ingredient in This is Your Life, which starts a new run this week. For Eamonn Andrews, it often means getting into disguise to spring the news on his subject, as our cover collection shows. "I dressed in a clown outfit, red nose and all, and waited in Charlie Cairoli's dressing-room while he was doing an ice show at Wembley," says Eamonn.

"I stood in a row of Marines dressed in Arctic gear when Jimmy Savile was inspecting the troops. You should have seen his face when I stepped out and broke the news to him!"

"When Joe Brown was the subject, he was rehearsing for Dick Whittington, so I took the place of his cat and hid the book under my fur. Joe had a real shock."

"I dressed as a busker and waited outside the recording studios to catch Peter Noone, but the most daring disguise of all was when I dressed as a station porter to catch Kenny Ball as he got off his train. I went up to him, said, "Porter, sir?" and he nearly fainted on the spot when he recognised me."

This Is Your Life - Article: Life Is Full Of Surprises