There’s a scene near the end of the iconic mid-1990s movie “The American President” in which Michael Douglas, as the president, asks Martin Sheen, the chief of staff, “Why are you always one step behind me?” To which Sheen responds, “Because if I wasn’t, you’d be the most popular history teacher at the University of Wisconsin.”

This Hollywood version of a president and his chief of staff (who in this case, a bit unrealistically, was also his best man at his wedding) is rooted in some truth, which is that the commander in chief relies on a right-hand man (there’s never been a woman in the role) to absorb some of the job’s slings and arrows.

On Friday night, President Trump announced that, after a week or so of rejections from potential candidates, he’d found someone within the administration to take the job on an “acting” basis. Mick Mulvaney, who reportedly was content in his role as director of the Office of Management and Budget, will now move to the West Wing to serve as Trump’s acting chief of staff.

The problem with appointing an “acting” chief is that it suggests it’s a temporary posting, which is “almost a contradiction” of the job’s intended role, said Christopher Whipple, author of “The Gatekeepers: How the White House Chiefs of Staff Define Every Presidency.”

Under normal circumstances, Whipple explained, the importance of the chief of staff to the president is enormous. He not only is the gatekeeper to the president, but also acts as his mouthpiece in the West Wing, on Capitol Hill and with foreign leaders.

“He’s the javelin catcher, taking all of the incoming flak for the president,” Whipple said. “He’s the person who executes the president’s agenda and, at the end of the day, he’s the person who can walk into the Oval Office and tell the president what he does not want to hear. That’s the most important thing a White House chief of staff does. It’s a huge job requiring an incredibly diverse skill set. It is not for the faint of heart. It remains a real question whether anyone can do that job for Donald Trump.”

It can be a thankless job in the best of times, so it’s little wonder Trump struggled to find someone who wanted to do it under less than ideal conditions. No one speaks for Trump but Trump. Being his chief of staff means standing idly by, being contradicted by your boss while having to defend the more outrageous things he says.

Whipple said Erksine Bowles, President Clinton’s chief of staff from 1997 to 1998 when a budget deal was negotiated with Congress, told him that people in Washington can sense when you don’t truly speak for the president. If that’s the case, “you’re no better than an overblown scheduler.”

“That’s been a problem with Trump’s presidency. No one believes that anyone can speak for him. When no one believes that the chief of staff speaks for you, then he can’t get anything done,” Whipple said. “The moment people suspect you’re a caretaker and don’t have real authority, you’re a dead man or woman walking in that job.”

There’s an added dimension to being invited inside Trump’s inner orbit: liability. Being that close to the president, even if just by physical proximity, opens that person up to intense scrutiny from the media, lawmakers on Capitol Hill and law enforcement. Trump’s presidency is headed for some rocky waters, and getting aboard now means being willing to go down with the ship.

“This is not only nearly mission impossible, but it’s also risky because any candidate has to think about lawyering up,” Whipple said. “This White House is headed into a world of trouble with a Democratic House, Mueller closing in and a brutal reelection. The White House chief of staff has to be ready for all of that.”