Prosecutors tried to poke holes in her version of what happened and the frustration was palpable for both the judge and the prosecution.
Many times through Officer Marysol Domenici's testimony, the judge interrupted her to say she wasn't adequately answering the prosecutor's questions and the answer's she did give about the scene that night -- prosecutors say don't match what's on the video.
Domenici told a jury that Grant grabbed her arm as he and his friends were detained on the Fruitvale station platform.
She describes a chaotic scene as noisy -- people were packed inside the cars, passengers shouted at her and they sang the song "lady cop" as she ran to help her partner, Tony Pirone.
Domenici testified, "I tell them multiple times to sit down. My memory was that Oscar did not sit all the way down. I know now that he did."
The officer drew her Taser, pointing it at one of Grant's friends. She said he repeatedly used profanity at her, called her the 'n-word' and threw his cell phone at her while she detained the rest of the group against the wall.
Grant's family says the officer, who was fired from the force in the wake of the New Year's Day shooting, stretched the truth to make the platform seem out of control.
"She felt that the platform was so chaotic and she thought it was a riot, and she thought she was going to have to kill someone that night," Grant's uncle Cephus Johnson said.
That dispatch fight call involving Grant was played for the first time in court. The dispatcher told officers there was no report of a weapon and Domenici said she didn't hear it.
About 40 to 50 people came off the train and onto the platform while officers were responding to the call -- that's the way Domencici described it.
Prosecutor David Stein showed the station surveillance video of Domenici running through an empty platform.
He said, "Isn't it true that, it is an attempt by you to exaggerate the conduct of those on the platform to justify the shooting?"
Domenici responded, "No sir, you don't see the people inside the train."
Domenici's testimony drew audible reaction from some Grant supporters in the audience, prompting the judge to hold the audience over and issue a warning. The surprise testimony of the day came from an expert called in by the prosecution to talk about how officers should stay calm and objective during arrests.