When the House select committee investigating the January 6 Capitol attack deposed British film-maker Alex Holder, it heard from a first-hand fact witness who inadvertently observed some of the darkest and most politically fraught days of Donald Trump’s time in office.

The new witness, who emerged late in the congressional investigation into the Capitol attack, had extensive personal access to Donald Trump and his family as the administration imploded in the post-2020 election period after the former president lost to Joe Biden.

Holder was there for it all: three sit-down interviews with Trump, including one at the White House, numerous other interviews with Trump’s adult children, private conversations among top aides and advisers before the election, and around the Capitol itself as it got stormed.

The second film-maker to cooperate with the panel – the first, Nick Quested, was embedded with the far-right Proud Boys group – in effect had a front-row seat to peer into the mind of the former president at the critical junctures in his efforts to retain the presidency.

The access to Trump, and listening to him and his inner circle, led him to suspect that the former president’s efforts to overturn the 2020 election would somehow culminate in some event at the Capitol on 6 January, Holder said in an interview with the Guardian.

“I wasn’t 100% sure, but it was sort of a feeling, so we prepared for that thing to happen,” Holder said. “The reason we thought January 6 was because, in Trump’s mind, the last-ditch effort was to stop the process” of Biden’s certification.

“That ceremonial process that takes place in Congress on January 6, he felt, was the last time where he could, in his mind, stop the election going to the wrong person, as it were. The rhetoric that was coming out was that the election was rigged, [that] we need to fight.”

Holder testified for about four hours behind closed doors last week about his roughly 100 hours of footage, used for an upcoming documentary titled Unprecedented, and turned over to House investigators the parts demanded in a subpoena compelling his cooperation.

The select committee was broadly interested in his recollections of the buildup to the Capitol attack, as well as his interactions with Trump and his family, Holder said, though he declined to discuss specific lines of inquiry or questioning.

The Guardian previously reported, however, that the panel zeroed in on phone calls among Trump’s adult children – including Don Jr and Eric – that Holder captured on camera at a campaign event on 29 September 2020 at the Trump international hotel that he gatecrashed.

The select committee is closely focused on the footage of the event – in addition to the content of the one-on-one interviews with Trump and Ivanka – because the discussions about strategies mirror similar conversations at that time by top Trump advisers.

What appears to interest the panel is whether Trump and his children had planned to somehow stop the certification of the election on January 6 – a potential violation of federal law – and to force a contingent election if Trump lost as early as September.

The event on the day of the first presidential debate at the Trump hotel that Holder gained access through Eric Trump, was unplanned, and reflected, according to Holder, his approach to filming everything he could, in case it proved to be consequential later.

Holder said he went into the one-to-one interviews with Trump and his children with a deliberately deferential approach and open-ended questions to ensure the exchanges did not come off as confrontational – including about whether Trump lost the 2020 election

“If I start pushing a guy who I know is not going to change his position, and then he throws you out of the room, then it’s all over,” Holder said. “I don’t need to argue and debate him because we contextualise his position with journalist interviews.”

“And also, this English guy from north London isn’t going to change Donald Trump’s mind about the election. Then we would have just wasted our entire hour together while I try to persuade him I’m right and he’s wrong,” Holder added.

The select committee has also been interested in Ivanka Trump’s interviews with Holder, according to a source familiar with the matter, since although she testified to the panel that she accepted that Trump lost the election, at the time, she told Holder the opposite.

Holder said he was not aware if that amounted to Ivanka Trump shifting her belief about the outcome of the 2020 election between her three interviews with him, but said he was surprised that she would effectively testify to the select committee that her father was wrong.

“That was surprising, because the three kids, at least with me, would always echo their father’s positions and support them,” he said.

The documentary broadly presents a portrait of Trump and his family that follows them through the tumultuous 2020 presidential campaign, when the children acted as campaign surrogates, the final months of the administration, and then months after the Capitol attack.

Holder said he interviewed Don Jr, Eric, Ivanka and her husband, Jared Kushner, before the 2020 election, and then went to the White House over the first weekend in December 2020 to interview the former president as well as Ivanka for a second time.

He said he did a second interview with Trump at Mar-a-Lago in Florida a few months after the Capitol attack, and then for a third interview with Trump at his Bedminster golf club a few months after that. He also interviewed Ivanka and Eric again after the events of January 6.

The documentary also features raw footage of the Capitol attack recorded by Holder’s director of photography, Michael Crommett, who filmed at the tunnel of the inaugural platform on the west side of the Capitol as the pro-Trump mob unsuccessfully tried to breach that door.

Holder said he additionally did a one-to-one interview with then-vice president Mike Pence, including a scene where Pence briefly reviews an email about the 25th amendment – which concerns the removal of a US president – which was privately discussed among senior White House officials in the wake of the Capitol attack.