Donald Trump appears on “The Tonight Show Starring Jimmy Fallon.” (Andrew Lipovsky/NBC via Reuters)

Anyone who expected Jimmy Fallon to go tough on Donald Trump during last night’s interview clearly hasn’t been watching “The Tonight Show.” Fallon is nothing more than free advertising for his celebrity guests. They come on, they play silly games, they act like good sports and they’re rewarded: The clips go viral and viewers marvel at how down-to-earth this or that A-list star is.

Trump’s visit was no different.

“I want to ask you, because the next time I see you, you could be the president of the United States,” Fallon said to The Donald during the show. “I wanted to know if there was something we could do that’s not presidential really …”

“Like what!?” Trump exclaimed as if all of this hadn’t been run by him before taping. “I’m not liking the sound of this.”

Fallon wanted to mess up the presidential nominee’s hair, and Trump — proving, as all Fallon guests must, that he doesn’t take himself too seriously — relented.

The host then went for it, while giggling. Trump didn’t look quite as pleased, but, hey, he did it.

The question is: What did people expect? If Matt Lauer can’t properly interview Trump, is it possible that the star of the Drew Barrymore rom-com “Fever Pitch” would do much better?

Maybe — if he had studied some old Letterman material.

Letterman interviewed Trump many times over the years. He wasn’t a presidential nominee at the time, true, but in 2015, it was pretty clear he was on the verge of announcing his candidacy, and Letterman responded to that by asking plenty of policy questions. Letterman never let Trump off easy, which explains why one of his “Late Show” clips made it into an anti-Trump ad.

That snippet came from a 2012 show when Trump went on a rant about how it was only a matter of time before China surpassed the United States as the world’s great economic leader. Minutes later, Letterman was holding some Trump brand ties (“sold at Macy’s and they’re doing great — number one selling tie anywhere in the world,” the mogul interjected) and asking Trump where they were made. Trump didn’t know.

“The ties were made in China,” Letterman said. Trump responded with an amused shrug.

The year before, Letterman and Trump had a bit of a feud after the late-night host told Dr. Phil that certain things Trump had said about President Obama smacked of racism. Trump was not pleased and said he wouldn’t appear on the show again without an apology. Letterman responded with something that wasn’t entirely contrite.

“I always want to believe the best in people,” Letterman said. “Maybe it’s not that he’s a racist. Maybe he’s just a guy that periodically says stupid things to get people’s attention.”

If it seemed like Letterman was going a little too easy on Trump, the host finished his on-air mea culpa with a zinger.

“I’m saying I would like him to be on the show,” he said. “It’s an element that we miss, especially with the campaign nearing fruition, to have a big dopey, puffy Donald Trump here.” Then, as he always does, Letterman made fun of Trump’s hair.

In any case, it was enough to get Trump back on “The Late Show.”

During the 2015 visit, Letterman introduced Trump as “America’s favorite real estate mogul and slumlord.” And when Trump came on, Letterman wasn’t about to let his guest say whatever he wanted and get away with it. When Trump talked about the rampant fraud and abuse tied to the Affordable Care Act, Letterman said, “Can you prove to me there’s been fraud and abuse? Let’s see the paperwork on that.” When Trump complained about how the United States looked like a third-world country, Letterman asked for solid ways Trump could fix the problem. He couldn’t offer any concrete ideas.

“It’s going to have a devastating effect on the economy,” Trump said of Obamacare at one point.

“I don’t know enough about it to say that’s not true,” Letterman said with his trademark up-to-no-good grin, “but I think that’s not true.”

Letterman was never one to back down. Fallon could learn a thing or two about proper interviewing by studying the former “Late Show” host. Come to think of it, Lauer could, too.

Related:

David Letterman taught us to ward off celebrity stupidity. But stupidity won.

Jimmy Fallon’s revamped ‘Tonight Show’: A playground where everyone gets a trophy