The 24-year-old Culkin confidently rejected suggestions that he might have been molested during his sleep without knowing about it and dismissed a prosecution display of sex magazines seized from Jackson's home, saying he used to keep Playboys under his own bed.
When defense attorney Thomas Mesereau Jr. asked Culkin if he was ever molested by Jackson, he replied, "Not at all."
"I don't think that jurors believe celebrities more than the rest of us, but I do think that jurors pay more attention when a celebrity is on the witness stand," notes CBS News Legal Analyst Andrew Cohen. "So I think the defense message of the day -- that Jackson was quite capable of keeping his hands off young boys -- probably got through clearly."
The actor's testimony was followed by a key decision in which the judge allowed the defense to play a videotape in which Jackson spoke of his love for children. The jury heard Jackson say, "They need to be held and loved and told somebody cares."
Culkin was the third young man to testify at Jackson's child molestation trial that as boys they slept with Jackson at his Neverland ranch and weren't molested.
Culkin testified that he and Jackson were drawn together by their common experience as child performers, although he laughingly noted that "it was not like a child actors' self-help group."
"Anyone who was a child performer," he said, "we keep an eye out for each other."
Jackson, 46, is accused of molesting a 13-year-old boy in 2003, giving him wine and conspiring to hold the boy's family captive to get them to rebut a documentary in which Jackson told an interviewer he let children sleep in his bed but it was innocent.
Culkin became part of the case when prosecutors were allowed to present testimony to suggest Jackson has a pattern of abusing boys. The testimony included an ex-chef who said he saw Jackson with a hand up Culkin's shorts as Jackson held the boy up to a video game.
Culkin appeared relaxed and answered questions directly during less than 90 minutes of testimony. Godfather to two of Jackson's children, Culkin has appeared in such Jackson productions as his "Black or White" music video.
Culkin testified that he was 9 or 10 years old when he met Jackson and that he slept in Jackson's bed several times between the ages of 10 and 14, sometimes with other boys as well. He said the sleepovers weren't planned and that he and others would just fall asleep when they were tired.
"I find that unlikely," Culkin said. "I think I'd realize that something like that was happening to me."
The prosecutor also asked if Culkin would have slept with Jackson at age 20.
"Probably not," said Culkin. "As you get older you begin to enjoy your privacy."
Culkin said he and Jackson often talked about missing out on their childhoods.
"It wasn't like therapy," he said, "but we talked about how these kind of things happen."
The young actor said he only learned of the allegations made about him when someone told him he should watch CNN.
At one point Culkin described Jackson as "very childlike."
"He liked doing the things we did. He liked playing the arcade games although he wasn't as good as we were," he said, drawing laughs.
Prosecutors seized on the "childlike" comment and another about how guests in Jackson's room were free to look at memorabilia.
The prosecution has shown the jury dozens of sexually explicit magazines found in his room during a November 2003 search, and have alleged that Jackson showed the material to his accuser and the boy's brother.
Asked if he thought it was childlike to have such material, Culkin said, "When I was 12 or 13 years old I had a couple of Playboys under my bed."
Culkin himself faces a court hearing next month on two misdemeanor drug counts in Oklahoma City. He was arrested Sept. 17, 2004, after police alleged found more than a half-ounce of marijuana and several tablets of the prescription medication Xanax in a car in which he was a passenger.
CBS News Consultant J. Randy Taraborrelli, a biographer of Jackson, says Culkin and Jackson have been friends for about 15 years. Jackson called Culkin personally and asked him to testify.
"His attorneys have been a little bit concerned that some of the testimony involving Macaulay Culkin in this trial really wasn't so critical to the proceedings," said Taraborrelli. "We heard from three former employees who talked about, you know, a kiss on the cheek, a pat on the butt, and some groping. They thought perhaps they didn't want to overplay that testimony. Plus they're concerned about the credibility of those witnesses who have a tabloid history."
"Michael Jackson actually called Macaulay Culkin when he saw the movie "Home Alone" in 1990. Macaulay was only 10 years old, and Michael was 32," Taraborrelli said. "They became very good friends, and Macaulay Culkin was at Neverland during the 1990s when all of the alleged activity was taking place there with some of the witnesses we've seen here."