After Ghislaine Maxwell took up residence in Jeffrey Epstein’s Palm Beach, Fla., mansion, the rules of the house became strict, a former employee of Mr. Epstein’s said on Thursday: See nothing. Hear nothing. Do not look him in the eye.

Testifying on the fourth day of Ms. Maxwell’s sex-trafficking trial, the former employee, Juan Alessi, who worked at the estate from around 1990 to 2002, said he had a “cordial” relationship with Mr. Epstein. But after Ms. Maxwell arrived in the early 1990s, he said, the dynamic changed.

Mr. Alessi said that Ms. Maxwell instructed him on how to speak to Mr. Epstein: “Mr. Epstein doesn’t like to be looked at in his eyes,” Mr. Alessi recalled Ms. Maxwell telling him. “‘Never look at his eyes, look in another part of the room and answer him.’”

The trial, which began on Monday in Federal District Court in Manhattan, is seen by many of his accusers and others as the trial Mr. Epstein never had, and prosecutors have sought to show that Ms. Maxwell, 59, played a pivotal role in her longtime companion’s sexual abuse of teenage girls.

Mr. Epstein, 66, was charged in 2019 with trafficking dozens of girls, some as young as 14, and engaging in sex acts with them. He was arrested in 2019, and killed himself a little more than a month later in a Manhattan jail cell.

Prosecutors have accused Ms. Maxwell of helping Mr. Epstein recruit his victims. She faces six charges, including sex trafficking, enticing and transporting minors for illegal sex act and three conspiracy counts.

On Thursday, prosecutors called an expert witness who testified about the concept of “grooming,” a technique that predators use to draw in their victims. Later in the day, Mr. Alessi took the stand and provided a rare glimpse of Mr. Epstein’s private life.

Mr. Alessi, 71, testified that he had seen two girls whom he believed to be underage at the mansion, including one, known only as Jane, who has already testified at the trial as a government witness. He said that Mr. Epstein’s employees were under strict orders about how to dress and about how they should address Mr. Epstein, Ms. Maxwell and their guests.

Prosecutors introduced into evidence excerpts from a 58-page booklet titled “Household Manual” that Mr. Alessi said dictated the rules at Mr. Epstein’s home.

“Remember that you see nothing, hear nothing, say nothing except to answer a question direct to you,” one section of the booklet read. “Respect their privacy.”

Mr. Alessi testified that he had taken the instructions as “a kind of warning.”

In her testimony, Jane said she had met Mr. Epstein and Ms. Maxwell in 1994, and she described Ms. Maxwell as acting like an “older sister” before she joined in the sexual abuse.

Mr. Alessi testified that he had first met Jane in 1994, and he recalled the first time she visited the estate with her mother.

“I don’t know exactly how old she was but she appeared to be young,” Mr. Alessi said. “I would say 14, 15.” (Earlier this week, Jane testified that she was 14 when she met Mr. Epstein and Ms. Maxwell, and that she was taken to Mr. Epstein’s house by a chauffeur, whom she remembered as a “sweet Latin-American man.” Mr. Alessi is from Ecuador.)

Mr. Alessi said either Mr. Epstein or Ms. Maxwell instructed him to pick Jane up from her home; he said he picked her up from school once. He described her as a “strikingly beautiful girl” who was “very pleasant.”

Mr. Alessi identified the second girl who appeared to be underage as Virginia Roberts. Ms. Roberts, who is now known as Virginia Roberts Giuffre, is one of Mr. Epstein’s most prominent accusers. She is not believed to be one of the four victims on whom the charges against Ms. Maxwell are based, and she is not expected to testify at the trial.

Mr. Alessi said he met Ms. Giuffre at Mar-a-Lago, former President Donald J. Trump’s South Florida club, on a day when Ms. Maxwell was visiting luxury spas in Palm Beach County. He said that Ms. Maxwell had asked him to stop the car when she saw Ms. Roberts.

By that afternoon, Mr. Alessi said, Ms. Roberts was at Mr. Epstein’s Palm Beach estate, and that she had returned often. Asked whether he knew where the girls went or what they did while they were at Mr. Epstein’s home, he said he did not.

“It was not my job to see where they were,” Mr. Alessi said.

But his job, he testified, did lead him to see other things. One of his tasks was to clean Mr. Epstein’s massage table, and Mr. Alessi noted that by the end of his employment, Mr. Epstein was receiving three massages a day. In her testimony, Jane said she was often asked to perform sexualized massages on Mr. Epstein as part of her sexual abuse.

Cleaning up after one such session in 1995, Mr. Alessi said he found a large sex toy — and then saw it again “at least four or five times” in the course of his work.

He said he did what he had been told to do: return it to a wicker basket as big as a garbage can that was filled with other paraphernalia. The basket, he said, was in Ms. Maxwell’s bathroom.