Diana Walker spent seven days in just as many countries with Hillary Clinton for this week’s cover story (available to subscribers here). But the cover image itself was taken in about five minutes, with just the light from a window, at Clinton’s desk at the State Department in Washington yesterday morning.

Photograph by Diana Walker for TIME

U.S. Secretary of State Hillary Rodham Clinton, October 26, 2011.

“She was at work at her desk, and I said, ‘Madame Secretary, look this way,’” says Walker, who has been photographing Clinton since January 1993. “She looked up at me, and then I said, ‘And now out the window.’ And bingo, that was the picture.”

Walker, who worked as TIME’s White House photographer for 20 years under Ronald Reagan, George H.W. Bush and Bill Clinton, says she prepared for her whirlwind week abroad with the Secretary of State in the same manner she did with former commanders in chief. “It’s not dissimilar,” she says. “You ride in the vans, jump out and run as fast you can to photograph her arriving. Then you line up and you’re herded into a room where the Secretary is meeting with whomever, and you make your pictures and then turn around and run out. And then you go to the next spot.”

In addition to standard press access during the trip, which began in Malta on Oct. 17 and ended in Uzbekistan on Oct. 23, Walker was also given time behind the scenes with Clinton, who she says doesn’t get enough credit for her “terrific” sense of humor—a characteristic she discovered while photographing the then-First Lady for TIME in 1997.

“I got in her limousine, and she was talking to her then chief of staff, laughing and having the best time,” Walker recalls. “That’s the image (below) we led with that year, in part, I think, because the editors were so impressed that it was another Hillary that they hadn’t seen. In public, she was much more straight-laced. In private, she could really kick back. I’m always eager to show that.”

Diana Walker

First Lady Hillary Rodham Clinton laughing it up with her chief of staff Melanne Verveer as they were returning to White House after delivering a speech at the University of Maryland in 1997.

That convivial atmosphere was around on this recent trip too, where Clinton often jokingly teased Walker for not keeping up. “Here I am lumbering along to catch up, while she has so much energy,” Walker says. “She wears me out! But it’s the most wonderful wearing out. She’s totally focused. But even with that laser focus, she has time to be funny and charming.”

Diana Walker was TIME’s White House photographer for 20 years, where she captured intimate moments with five presidents. You can see more work in her book The Bigger Picture: Thirty Years of Portraits and in our coverage of Steve Jobs. 

Feifei Sun is a reporter at TIME. Find her on Twitter at @Feifei_Sun or on Facebook. 

MORE: Read TIME managing editor Rick Stengel’s exclusive Q + A with Hillary Clinton.