Times change - and so, in the same kind of way, do spies. The first time we saw him in The Hunt for Red October, Jack Ryan was young, new to the CIA and played by Alec Baldwin. Then he got a little older, a little further up the Agency ladder, and became inextricably associated with Harrison Ford, who played the role in Patriot Games and Clear and Present Danger. But now, In The Sum of All Fears, eight years on from his last big-screen outing, he has suddenly got younger and looks remarkably like Ben Affleck.

Affleck, who has more or less cornered the market in intelligent younger action heroes following Armageddon, Reindeer Games and Pearl Harbor, was understandably nervous about taking on the mantle of the most famous CIA agent in movie history.

“I admire and respect both Alec and Harrison Ford,” he says. “I sincerely like the movies they did and, if one of those guys were doing this film, I’d be buying a ticket. Actually, I was working with Alec [on Pearl Harbor] when The Sum of All Fears was offered to me. We discussed my taking on the role of Ryan, and he was very encouraging. Then I called Harrison to get his blessing, so to speak, and he was also very supportive.”

As a dedicated follower of a character whose films have so far grossed nearly $575 million worldwide, won an Oscar and been nominated for two more, Affleck would also have known something else: that Ryan’s creator, Tom Clancy - who to a degree based his hero on his own rise through the ranks of the CIA - had clearly plotted a career path for his character.

In the first book, he is a wet-behind-the-ears young analyst who has just joined the Agency. Slowly, however, as the series progresses, he gets promoted, rising through the ranks to become first deputy director, then director of intelligence. By the time we leave him, he is ensconced in the Oval Office.

But The Sum of All Fears needed to be contemporary, because it is about the threat to the US from the proliferation of nuclear weapons, both in a handful of ‘rogue states’ and as a result of the warheads that went ‘missing’ after the collapse of the Soviet Union (according to official records - and we’re talking the real world here - 164 of these are unaccounted for). So writers Paul Attanasio (Quiz Show, Donnie Brasco) and Daniel Pyne (Pacific Heights, Any Given Sunday) simply made Ryan young again. You can do that in the movies.

Here, the CIA - which Ryan will one day run - is headed by William Cabot (Morgan Freeman, an actor who has already played the President and already had to do what he could to save America from disaster in Deep Impact). Faced with a change of government in Russia and a threat to the US, Cabot takes the classic agency route of sending his best agents out into the field.

Thus, young Ryan’s first assignment is to find the people who may be responsible for the threat to his country, and he is placed in the care of veteran agent John Clark (Liev Schreiber).