As a documentary filmmaker, Nick Quested has made a career of telling other people’s stories — many of them under harsh conditions. Over the years, he has made films about soldiers under fire in Afghanistan and the inner workings of a Mexican drug cartel.

But on Thursday night, when he appears in front of the Jan. 6 committee, Mr. Quested, 52, will tell a part of his own story as he is called upon to relate where he went and what he saw when he was embedded with the far-right group the Proud Boys before and during the Capitol attack.

Mr. Quested’s testimony will begin with a brief opening statement, according to a person familiar with the matter, and will include several segments of video filmed as he moved with the Proud Boys from the early morning of Jan. 6, 2021, through the breach of the Capitol.

Mr. Quested was also present with a camera on the day before the riot, when the Proud Boys’ former chairman, Enrique Tarrio, met in an underground parking garage near the Capitol with Stewart Rhodes, the leader of the Oath Keepers militia. (Both men have been charged with seditious conspiracy in the attack.)

Mr. Quested began spending time with the Proud Boys after the election as part of a project meant to explore the increasing polarization and extremism in American politics. He is also likely to testify about the Proud Boys’ actions at two pro-Trump rallies in Washington that preceded Jan. 6: one on Nov. 14, 2020, and the other a month later on Dec. 12.