"It was just for fun, just exciting," she testified in 4th District Court on Wednesday. "Just for on the side."
But she said "circumstances changed completely" after Michele MacNeill's death.
MacNeill needed support, she said, so he decided to bring her into the family but he staged a meeting, pretending like she was only an acquaintance and brought her into his family's home as a nanny.
"What Martin told me was that he had felt very sorry that we were involved prior to his wife's death," she testified. "He needed help, he needed assistance. He was embarrassed about our relationship, but thought that I could come and act as help. But he wanted to have me meet his family with the best foot forward."
Martin's adult daughter, Rachel MacNeill, testified Wednesday that the first time she met Willis, it was a few days after her mother's death, and she was sitting outside of the Church of Jesus Christ of Latter-Day Saints temple in American Fork with her father.
They were on a bench, talking about their prayers concerning hiring a nanny to care for the four younger children, Rachel MacNeill said, when a woman interrupted them, saying that Martin MacNeill looked familiar.
"I'm so sorry for your loss and the grief you've been through," Rachel MacNeill said the woman told her. "My name is Jillian."
The woman named Jillian who Rachel MacNeill said she later came to find out was Willis talked to her about nursing school, and said she had pamphlets in her car about the program she was in.
"I just said, 'You know, I'm just trying to breathe right now.' " Rachel MacNeill testified. "I was just not in the mood to chat with her about nursing school. I wasn't interested in being a nurse."
Willis, 36, testified that the meeting was MacNeill's idea a ruse to meet with the MacNeill children without the pretext of being their father's mistress.
"There was not a script," she said. "It was more that he thought the place was a nice place and he thought it would be a calming environment."
Within weeks, Willis had moved into the MacNeill's home, hired as the "nanny." Willis insisted she cared for the children, but Rachel MacNeill said she was unlike any nanny she'd ever seen.
"I expected to see what I've seen before, that their main focus was the children," she testified. "That they were there to work. That was their job making sure the children were taken care of, doing things with them. Not just sitting and staring at my dad."
MacNeill, a 56-year-old doctor, is accused of killing his wife in April 2007, in order to free himself to continue his affair with Willis.
MacNeill allegedly gave 50-year-old Michele MacNeill a deadly mixture of prescription drugs after she came home to recover from cosmetic surgery in April 2007.
Wednesday marked the fifth day of MacNeill's preliminary hearing for MacNeill.
On Thursday, Judge Samuel McVey will determine if there is enough evidence to order MacNeill to stand trial on charges of first-degree felony murder and obstruction of justice, a second-degree felony.
MacNeill remained emotionless and tight-lipped while his oldest daughter testified against him, as he had with every other witness.
But when Willis who testified as part of a plea deal from an earlier identity fraud conviction took the stand, he cracked small smiles when the two would lock eyes occasionally during her testimony.
Though Willis described MacNeill as "wonderful" and thought he was "many things that would make a wonderful companion," she said she never expected a relationship from him, and they never talked about him getting a divorce or cutting Michele MacNeill out of the picture.
But old roommates of Willis countered her testimony, saying that she had a photo of Michele MacNeill in her bedroom, and had once talked of cutting the woman's brake lines to get rid of her.
"I was scared to death of Gypsy when I moved out," Michele Savage testified. "I'm still scared of her."
But Willis testified that she never talked about cutting brake lines, and that she had lived with Savage and her daughter before she ever dated MacNeill.
Savage and her daughter, Brandy Smith, testified that Willis had spoken about a man she dated named "Neil," who was married, and was a doctor and lawyer. Smith said she was certain she saw MacNeill come to the house once, before Willis gave her $200 and told her to take another roommate and her son and go find something to do for a few hours.
Despite the testimonies about the affair and MacNeill's behavior in the days before and after his wife's death, MacNeill's defense team, Susanne Gustin and Randy Spencer, said the case will likely come down to scientific evidence.
"If you can go past the sensational nature of this case, it's going to come down to cause-of-death," Gustin said.
How Michele MacNeill died seems to be somewhat of a mystery, even to medical examiners. The Utah medical examiner's office has never ruled Michele MacNeill's death a homicide.
After an autopsy in 2007, assistant medical examiner Maureen Frikke who died in 2008 ruled the manner of Michele MacNeill's death was "natural," the result of "chronic hypertension and myocarditis, which are capable of causing acute unexpected arrhythmia and sudden death."
But investigators say MacNeill called Frikke multiple times, giving misleading information that included how her body was found.
In 2010, in a new investigative report, Chief Medical Examiner Todd Grey changed the cause of death to the combined effects of heart disease and drug toxicity. The manner of death was changed to "undetermined."
Grey said there was some evidence that Michele MacNeill may have drowned, pointing to prior testimony from medical responders who said that while undergoing CPR, the woman coughed up several cups of water, some of it pink and frothy.
That may be consistent with water and air found in the lungs, Grey testified Tuesday, but said he did not see the microscopic findings in the lungs that indicates a drowning occurred.
"I didn't find enough physical evidence compelling enough to say, 'Yes, I know this was a drowning,' " he testified.
However, Dr. Joshua Perper, a medical examiner from Florida asked by prosecutors to review the case, said he disagreed with both of the Utah medical examiners, and said he thought Michele MacNeill drowned. He said he believes the drugs found in her system or her pre-existing health conditions may have contributed to her death, but neither were the cause.
"In my opinion, those drugs may have contributed in some way to the death, making the person more prone to drowning, or more prone to a person drowning the [victim]," he testified on Wednesday.
Perper said that although he felt Michele MacNeill drowned, he still ruled the manner of death as "undetermined," because it could not be determined whether her death was accidental or homicidal.
The question of whether Michele MacNeill would have taken the mixture of drugs and committed suicide was also presented to Perper, but he disregarded the theory, saying that MacNeill's adult daughter reporting her mother was fearful of her husband killing her would indicate that she was not showing suicidal tendencies.
"If you are contemplating suicide, and you wanted to kill yourself, you wouldn't be fearful of someone killing you," he said. "… It may facilitate your wishes, it doesn't threaten."
Willis has been prosecuted in both state and federal court for identity fraud. After pleading guilty, she spent 21 months in a federal prison, six months in the Utah County jail and is still on probation in the state case, according to court records.
Willis, with the help of MacNeill, obtained false military identifications and a Utah identification card. MacNeill also helped Willis open numerous bank accounts under a false name.
MacNeill, who also pleaded guilty in both state and federal court, was sentenced in 2009 to four years in a federal prison.
Meanwhile, Utah County prosecutors are currently prosecuting MacNeill on felony charges of forcible sexual abuse and witness tampering for allegedly putting his hand down the pants of an adult relative in 2007, and then asking her to sign a statement saying he did not touch her.