More complicated trips include teaming up with a local field offices, housed in every U.S. state, the District of Columbia, Guam, Puerto Rico and foreign countries including Brazil, Italy, Russia, South Africa and Colombia. Agents at these bureaus work on regional investigations until they get a call that the president is coming through; then they transition to assisting in advance work and man a post during the actual trip.
Advance teams also consult with local mental hospitals. Hinckley, for example, had been treated for depression at a psychiatric center prior to his assassination attempt on Reagan, so agents want to know who has been recently discharged and whether that person could be a potential threat.
"Most of the planning now, as opposed to Dallas in 1963...is 'How do we prevent any possible situation from arising?'" says author Jeff Robinson. He wrote "Standing Next to History" with former Secret Service special agent Joe Petro, who guarded Reagan for four of his 23 years with the agency.
"There is reaction, absolutely," Robinson says. "But there is so much work being done preventatively."
The best defense, as the saying goes, is a strong offense.
OFFENSE AND DEFENSE
"Things didn't change immediately, it took a considerable period of time," says Hill, who returned to the Secret Service on Jacqueline Kennedy's protective detail until 1964. He documents that time in his book, "Mrs. Kennedy and Me: An Intimate Memoir." His latest work, "Five Days in November," will be released later in November.
The immediate changes focused on direct security, he says. Presidents could no longer drive through densely populated towns with the car's top and windows down, or exit it without some sort of canopy or cover. And people who have not been thoroughly screened cannot directly approach him.
The Secret Service, in turn, had to expand its coverage of public officials who would not be contained. Agents famously had to accompany Reagan on his beloved horseback rides. Dan Quayle and his family were avid skiiers, requiring a Secret Service escort on the slopes. Secret Service training produces good runners, but agents had to adapt to George W. Bush's brisk regimen while simultaneously scanning for threats.
But the largest subsequent change was in training.
"[The assassination] made us more determined than ever that it was not going to happen again," says Hill. "We worked extra hard, more diligence."
Training for agents now includes a rigorous course at the Federal Law Enforcement Training Center in Glynco, Ga., and then at the Secret Service's own James J. Rowley Training Center on 500 acres just outside of D.C. The 31-building facility is not just limited to recruits. Veteran agents return frequently for refresher courses or other training to further enhance and hone their capabilities.
It is named for the Secret Service director from 1961 to 1973, credited with pushing these institutional changes to take on an increasingly dangerous world.
"Threats come in every day," says Emmett. "Today, any sort of threat is investigated. Nobody wants to be the agent who is there when they fail."
"If we could boil this down to a simple answer to that question, it would be training," he says. "Just training."
And this training must be as intensive as it is continuous, Emmet adds. That is the attitude today.
BLACK SUNGLASSES AND AN EARPIECE
As one former agent says, "Sometimes the projection of power and strength is a deterrent as much as anything else."
The iconic image of a Secret Service agent perhaps has not changed significantly since 1963. Most could pick out the telltale well-groomed agents, always with suit jackets unbuttoned for easy access to their sidearms, and a curly wire protruding from one ear.
But unlike 1963, the full scope of the agents on-scene has changed. Presidential motorcades now include standard armored cars with multiple limousines to conceal the true location of the president. At least one SUV has windows rolled down to reveal the muscular operatives in black tactical gear bearing heavy duty machine guns -- the trademark look of elements of the Secret Service Counter Assault Team. Everyone who gets near to the president must first go through magnetometers, and are greeted with a perimeter of tactical agents guarding the premises with canine units and rooftop snipers.
Corrected on : Corrected, 11/19/13: An earlier version of this article misidentified Jeffrey Engel, a professor at Southern Methodist University.