How does a researcher uncover what really went on during top secret conversations between the Allied leaders Churchill, Roosevelt and Stalin more than 60 years ago?
Our aim for the series 'World War Two: Behind Closed Doors' was to tell the history of this period in a way that had never been done before on television.
We wanted to reveal the secret meetings held between Joseph Stalin, Winston Churchill and Franklin Roosevelt, and to show how the decisions that these three men made were to effect the lives of millions of people.
The research challenges that this presented us with were enormous. How could we possibly know what was said in these meetings, and how could we be sure that the sources we found were accurate?
It is crucial to the integrity of the series that each scene of drama reconstruction was meticulously sourced. Each line of dialogue in the series is, as far as we can possibly know, what was actually said at the time.
The behind-the-scenes conversations between the three leaders that are re-enacted are not embellished for dramatic effect. They are a true reflection of the characters of the three men and the relationship between them.
A good starting point for researching these events is the National Archives in Kew and other state archives across Europe and the United States. Extensive official minutes were made of many of the meetings we wanted to dramatize.
The American minutes, found in the 'Foreign Relations of the United States' volumes are, incredibly, all available on the internet. Official minutes often record the time of the meeting, who was present, and what was said, but they can also give an indication of the mood of the discussions.
For example, in a meeting between Churchill and Stalin in August 1942 the official minutes show that Stalin 'had begun to look very glum'. A little later it was noted that Stalin's 'glumness had by now much increased', and he continued to 'become restless' and look 'glummer still'. Details such as these are invaluable for portraying the meeting accurately on-screen.
But as with all historical sources the official minutes of meetings must be treated with caution and assessed for reliability. We consulted every available source for each scene, from diaries and logs to memoirs and photographs, in order to build as clear a picture of events as possible.
For scenes that had fewer sources to refer to, or where reliability of information was in doubt, we had particular guidance from our academic consultants in Britain, Russia and the US.
These additional sources often reveal more than the official minutes were willing to record. On a visit to Moscow in October 1944, Churchill suggested to Stalin that between them they should discuss the division of Europe at the end of the war.
Churchill jotted down some suggestions which he handed to Stalin saying: 'This is rather a naughty document. The Americans would be shocked if they saw how crudely I have put this.'
While details of this meeting are recorded in the official minutes, as well as in Churchill's own volumes on the history of World War Two, this particular attempt by the prime minister to appeal to Stalin's ruthless nature was only recorded in notes of the meeting made by the British ambassador in Moscow.
Notes of meetings can also reveal information that other sources such as photographs fail to provide. Take the moment in August 1939 when the signing of the Non-Aggression Pact between the Nazis and the Soviets was caught on camera. Just before the picture was taken, Stalin and the two foreign ministers, Joachim von Ribbentrop and Vyacheslav Molotov, celebrated with a drink.
When asked if he would allow a photograph of the occasion to be taken Stalin replied: 'My only condition is that the empty bottles are removed beforehand, otherwise people might think we got drunk first and then signed the agreement.'
Details such as this, which reveal the character of the Soviet leader, were recorded in notes made by a German diplomat present at the time, and only recently unearthed in an archive in Germany.
Personal diaries of people who were present at behind-the-scenes meetings also provide valuable sources of information. Two that we found particularly useful were the diaries of British diplomats Sir Alan Brooke and Sir Alexander Cadogan.
As they are daily records of events they often provide useful pieces of practical information neglected in other sources, such as the time of day and descriptions of meeting rooms. But their greatest value comes from their candid remarks about the three wartime leaders and the dealings between them.
During a visit to Moscow in December 1941 Cadogan noted: 'Difficult to say whether S. [Stalin] is impressive. There he is - a greater Dictator than any Czar (and more successful than most). But if one didn't know that, I don't know that one would pick him out of a crowd. With his little twinkly eyes and his stiff hair brushed back he is rather like a porcupine.'
Though both diaries were published some years after the war they were written at the time as personal records, and offer a unique insight into the opinions of the two authors without the complication of hindsight.
The diary of Joseph Davies, former US ambassador to Moscow, kept in the Library of Congress in Washington, includes fewer observations as frank as Cadogan's, but it does record conversations in great detail.
President Roosevelt, in an attempt to curry favour with Stalin, suggested a meeting between the two of them in 1943 without the British. Davies was sent to offer the proposal to Stalin, and his diary gives a detailed account of the discussions they had and clearly shows Roosevelt's attempts to belittle Britain's power.
This becomes all the more shocking when Churchill eventually hears of the suggested meeting and Roosevelt baldly lies to him that he 'did not suggest to Uncle Joe that we meet alone.'
As with all sources Joseph Davies' diary must be looked at critically, and it must not be forgotten that it represents just one view of a meeting between a number of people. We were incredibly fortunate on this project to have benefited from the opening of archives in the east since the fall of Communism.
Documents that were previously inaccessible became available to us, and these often made us look at the history in a new light. Many of the meetings portrayed in the drama reconstructions involve people from two or more countries, and the differing records of events can be very revealing.
During Molotov's visit to Washington in 1942, American records mention in passing that Harry Hopkins, a special emissary to Roosevelt, visited Molotov in his bedroom the night before the main meeting with the president.
A Soviet record of this encounter - minutes made by Molotov's interpreter, Pavlov, published for the first time in 1997 - reveal that Hopkins offered the Soviet foreign minister advice on how to 'handle' the meeting the following day. It is perhaps understandable that any mention of the conversation was omitted from Hopkins' records.
The 1990s in particular brought a wealth of material into the open. This included many of Stalin's documents, like the log book of visitors to his office, but historians have found it increasingly difficult to gain access to some files in the post-Yeltsin years.
One of the central themes of the series 'World War Two: Behind Closed Doors' is the Katyn Massacre, which not only demonstrates Stalin's cruelty, but also the balance of power in his relationship with Churchill and Roosevelt.
Soviet guilt for the murder of over 20,000 members of the Polish elite in 1940 was only admitted for the first time by Mikhail Gorbachev in 1990, and it wasn't until 1992 that the document proving Stalin's complicity in the crime was made public - over half a century after the crime was committed.
While proof of Soviet guilt for the murders at Katyn has been known for 16 years, we have been able to show how those murders took place through drama reconstruction in a way that has never been done before.
Russian prosecutors conducted an investigation in the early 1990s, and we gained access to an interview with a man who helped to arrange the killings personally.
He described the executioner wearing a 'brown leather apron' and 'brown leather gloves with cuffs over the elbows', and told of how 'they covered the doors to the shooting cells that led to the corridor so the sounds of the shootings couldn't be heard'.
Such testimony gives a unique insight into the methods used to carry out the massacre and brings into focus the true cruelty of the crime.
Every effort was made to ensure that not just the dialogue, but the sets, costumes and the appearance of the actors were as accurate as possible. In some cases we were helped by the existence of photographs of events, such as the signing of the Non-Aggression Pact already mentioned.
Photographs were a huge help in getting the details of the drama scenes right, and in this instance we were even able to know exactly who was standing where as the pact was signed.
Just as the fall of Communism allowed greater access to eastern archives, it also allowed people to speak more freely about life under Stalin's rule. Throughout the making of this series we were fortunate enough to meet many people from the former Soviet Union and Eastern Europe who were involved in the war.
Their testimony peels away another layer of the history to show the widespread effects of decisions made in the Kremlin.
Many of them were speaking about their experiences on camera for the first time, such as the lady from Eastern Poland who was accused of working for a resistance group. Her moving description of her interrogation and torture gives a stark appreciation of life under the rule of the Soviet dictator.
The limitations of all historical sources must be recognized. Even a video camera can only ever give a view of a room from one angle. But through our meticulous research and study of the sources we can be sure that the words spoken in the drama reconstructions in the series are, as far as anyone can know, a true representation of the behind-the-scenes meetings as they actually happened.