Transcript for Martin MacNeill's Mistress Takes the Stand
But we begin this half hour with the explosive testimony at the so-called bathtub murder trial from the woman at the center of the case. Taking the stand, the mistress of the man on trial, who maintained his innocence. She is giving her side of the story. Aditi roy has been following the trial from day one. Reporter: Gypsy willis did not shy away from cameras as she strode into court. PROSECUTORS say MacNeill killed his wife of nearly three decades to continue the affair with willis. We would see each other a couple times of month. Reporter: Macneill is accused of overdosing michele with painkillers while she recovered from plastic surgery. MacNEILL SAYS HE IS INNOCENT. Michele's heart disease was either the cause or the contributing cause to her death. Reporter: But more than a year before michele's death -- did the relationship become sexual? It did. Reporter: Willis told jurors she moved into an apartment, PAID FOR by MacNeill. And she went to michele's funeral. Weeks later, willis moved into the MacNeill home, as the live-in nanny. Was there a question that you would be hired as the nanny? I don't believe martin would have had me come in if the children objected to me. My dad was cooking. She was sitting there, staring at my dad. Things like that. The children were taking care of themselves. Reporter: Ada, the youngest MacNEILL CHILD WAS JUST 6, WHEN She discovered michele unconscious in the bathtub. The judge ruled that the little girl, now 12, could go on the stand. But her testimony will be limited, since she was interviewed by older daughter, alexis, who believes her father is guilty. Now recess for the weekend. Reporter: Gypsy is expected to continue her testimony on tuesday. Providing more answers that could decide the fate of her former lover. For "good morning america," aditi roy, abc news, los angeles.
This transcript has been automatically generated and may not be 100% accurate.