In this video from 2005, Donald Trump prepares for an appearance on "Days of Our Lives" with actress Arianne Zucker. He is accompanied to the set by "Access Hollywood" host Billy Bush. The Post has edited this video for length. (Obtained by The Washington Post)

The journalistic lessons of this presidential campaign keep rolling in. Some of them are painfully obvious. They include:

Check your mailbox. (Donald Trump’s tax return may be waiting there, as Susanne Craig of the New York Times discovered some weeks ago.)

Answer your phone. (Someone may offer to give you a mind-blowing video of Trump describing his own disgustingly lewd behavior, as David Fahrenthold of The Washington Post found out Friday.)

And research your own company’s archives. If you find something extraordinarily newsworthy there, don’t sit on it at a crucial moment in the nation’s history.

The video sent to Fahrenthold was part of a taping session of NBC’s entertainment-magazine show “Access Hollywood” from 2005. A producer from “Access” dug it up recently and the show was vetting it before broadcast, according to an NBC spokeswoman I talked to Saturday.

Why didn’t NBC News do the basic journalistic work of working through its own company’s archives on Trump months ago?

It may have to do with the split between the entertainment and the news divisions — both “The Apprentice” and “Access Hollywood” are products of NBC’s entertainment side, not its news side. NBC’s news reporters are often excellent — Katy Tur, for example, has covered the Trump campaign with admirable intelligence and assertiveness, and has taken a lot of flak from the campaign for doing so.

NBC’s entertainment division has had a long history with Trump because the network was the home of “The Apprentice,” which made him a reality-TV star. Separately, Trump did hundreds of interviews with NBC for “Access Hollywood.” That means there are two separate caches of Trump-on-tape that could yield news.

The video that surfaced Friday, in a story broken by The Post’s Fahrenthold, is just one tiny piece of what NBC has recorded with Trump over the years.

That tiny piece, though, has rocked the presidential campaign. Trump’s casual banter, in which he brags about sexually assaulting women, could end up as the death blow to the campaign.

Given its importance, why did it take a tipster to The Washington Post to force this material out into public view immediately? (It took Fahrenthold only five hours from phone call to publishing his story.)

What else is in the myriad hours of recordings surrounding “The Apprentice” and “Access Hollywood”? (The video that surfaced Friday was the result of a “hot mic” — it wasn’t meant to be recorded.)

And shouldn’t the “Access Hollywood” material have been turned over to the news division, given its clear importance to national politics?

NBC has been cryptic on the subject. My requests for interviews with top NBC brass Andrew Lack and Steve Burke went unanswered.

Ali Zelenko, senior vice president for communications, told me that NBC News has no special access to the “Apprentice” recordings.

“We don’t own the footage,” she said in an email. “We’re pursuing it along with every other news organization reporting on this story.” She did not respond to my question about who does own it. It may belong to the reality show’s longtime producer, Mark Burnett.

But what about the separate “Access Hollywood” recordings, which the entertainment show’s host, Natalie Morales, made clear are available.

“In the course of reporting on Mr. Trump, we have reviewed much of our own footage,” Morales said on the air. And an “Access Hollywood” spokeswoman said in a statement: “We have combed through every interview we have done with Mr. Trump over the past 20 years and, at this time, we have not uncovered any other footage that rises to this level.”

One troubling factor is that “Access Hollywood’s” segment on Trump’s horrifyingly lewd comments — despite their stunning news value — was not scheduled to appear immediately, a spokeswoman confirmed. That only changed when news of The Post’s imminent article forced producers’ hands; they aired it Friday night, after all.

Why this material was left in the hands of an entertainment show, and not the news division, was something I couldn’t get an answer to on Saturday. And when would it have run? Since the segment wasn’t scheduled for Friday, the earliest possibility on the entertainment magazine show would have been Monday.

That would have made its contents yet unknown for the second presidential debate on Sunday night. (Notably, that’s the only one of the three debates in which there is a female moderator.)

The segment was “being carefully vetted,” said Hilary Smith, senior vice president for communication at NBC. But when it became clear that The Post would write about it, “Access” decided to air what they had been working on.

Jay Rosen of New York University told me Saturday that it shouldn’t have taken very long to make the video public.

Because it was NBC’s own recording, there should have been no question about authenticity, and, as he put it, “the tape is the story.” Reaction from the campaign was the only thing left to do. “That’s an hour,” as he put it.

What’s more, NBC’s Billy Bush — a former co-host of “Access Hollywood” and now a co-host of its “Today” show, which is on the news side of NBC — knew about it for 11 years but apparently didn’t see it as newsworthy.

Is there now a news-side reporting team at NBC going through all of the network’s Trump recordings and trying to get access to its own network’s “Apprentice” recordings?

And why didn’t this kind of digging happen many months ago — by the networks’ news reporters?

There is significant public interest in knowing the answers to these questions. NBC owes it to the American electorate to answer them clearly.

Without adequate answers from the top, made publicly, it may not be unreasonable to conclude that one of the nation’s largest news companies simply wasn’t doing its job.

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