But then the shooting happened. She was struck in the arm and face by shotgun pellets by a gunman who attacked the crowd during the midnight screening of "The Dark Knight Rises."
One pellet entered her skull, traveled with a slight curvature on its trajectory and managed to miss any vital areas of the brain. In the last seven days, she has had brain surgery, been visited by actor Christian Bale and recovered enough to walk around, scold her mother for trying to help her with putting toothpaste on the brush and asked for a meal of steak and broccoli at the Aurora Regional Medical Center.
"On the third morning, I came to the ICU and I thought she was sleeping," her mother, Kim Anderson, said. "I moved close to her face and her eyes popped open. She said, 'When am I going to get out of this place?' "
Turns out, that would be Friday exactly one week from the tragedy that killed 12 and wounded 58.
'The tool she needs' • She is among the last theater shooting victims still at The Aurora Medical Center. But she has become famous for the "miracle."
That is the term used by most everyone who has seen the X-rays and heard the explanation of how a shotgun pellet managed to hit her at the precise angle and pass through her brain in a way that didn't do any severe damage.
"If the bullet had wavered a millimeter in any direction, she would have likely either died or been severely injured," said Dr. Michael Rauzzino. As the neurosurgeon who operated on Anderson at the Medical Center of Aurora, he, too, calls it "a miracle."
At the hospital Thursday, Kim Anderson sat in the main lobby after spending six hours with her daughter and thought about the week that had passed. About how Petra looked when she saw her before going into emergency surgery after she and Petra's sister, Chloe Anderson, rushed to the hospital after the phone call from police saying there had been "an incident."
Anderson said when she saw she was getting ready for surgery, she was scared.
"That's about when I lost it," Kim Anderson said. "I was thinking that she's going to be a musician. Her brain that's the tool she needs."
Anderson paused, her eyes brimming with tears. A family friend, 23-year-old Andrew Robyler, kneeled next to her and put his arm around her. Anderson's voice cracked.
"I thought: Will I ever get to talk to my Petra again?"
Petra's sacrifice • Before the shooting, the Anderson family had been rocked by news that Kim Anderson's breast cancer was aggressively back after being in remission for 18 months. It had spread to her major organs. She said doctors told her she had several months left to live.
She is attempting to qualify for a clinical trial in Texas. The news of the cancer's return in June pushed Petra into telling her mom she was going to pass up a music composition teaching post at the University of Maryland this fall to spend time with her. "I'm not going," said Petra. "I want to do whatever I can with you for as long as I can."
Kim Anderson would have none of it.
"I told her if she stayed, I would just die today."
Anderson admits she is weak already from the cancer. The stress of the past week has kept her from getting much sleep. Eating a restrictive diet amid irregular hours has taken its toll as well. She said it's hard for her to conceptualize that it's been a week since the shooting occurred. Her voice is soft and weary when she speaks.
The family's life for the week has been six-hour routines. Kim Anderson stays with Petra from 6 a.m. to noon, with Hogan, Petra's boyfriend of two years, coming in to play music in the morning. From noon to 6 p.m., it's a rotation of family and friends, including Chloe Anderson. From 6 p.m. to midnight, Petra's father recently divorced from Kim takes the shift.
At Kim Anderson's house, family and friends are cleaning and disinfecting the rooms for her return after a stop in a rehabilitation facility.
But with the Andersons, the conversation always returns to music.
They were excited to learn that her alma mater, the University of the Pacific, is hosting a benefit concert in late August to help with the medical bills. The program will consist of music entirely written by Petra. Another benefit concert will happen at the Interlochen Center for the Arts in Michigan.
The 'incident' • Chloe Anderson is exhausted and picking at a chocolate chip cookie with a dollop of ice cream at a local pizza joint.
It's Wednesday night just five days after the shooting and her voice is strained from exhaustion. Robyler, the family friend who came from Texas to help out, is the schedule master for the family, and he is anxious for her to get some rest.
But the 25-year-old has things to say. She replays in her mind the moments when she learned about the shooting and how she was told what her sister's medical status was.
" 'Breathing' is the word they used," Chloe Anderson said.
The worst part, she said, was when she was going into surgery. She heard the words from the operating team that sounded almost surreal to her.
" 'Say whatever you need to say to her,' " Anderson recalled. "I really didn't know what to say. So I just said, 'I love you and I'll see you when you're done.' "
When she did see her afterward, there was relief. She went home and cried tears of exhaustion and relief. Then, in a defiant move to show the shooter that she wasn't going to be bullied or fearful, she went to a movie theater to watch "The Dark Knight Rises." A different venue a few miles away, but the same theater number where the shooting occurred Theater Number 9.
She sat in the front row, too.
Anderson said it felt OK to her though a few scenes made her a little jumpy. She tried to imagine what her sister saw. When her mother asked Petra what she remembered days after the shooting, it was a groggy answer aided by trauma and painkillers.
"There was smoke. Then Batman came off the screen and started shooting people," Kim Anderson recalled.
Great expectations • Kim Anderson has little doubt that her daughter, who composed music using a computer but also would use pen and paper while hopping from keyboard to piano, will write a piece about the theater shooting. Already, Petra Anderson had written a piece called "Salute" while at the University of the Pacific that featured seven vocalists and a full orchestra that played to a slide show based on letters written during every American war.
Chloe Anderson said her sister was always creating music compositions many times from concepts swirling in her head. One, she remembered, was based on placing an arm on the piano keys and hearing the crash of the discordant notes. It explored the decay of those notes as they left the piano. She called it "Penumbra" a shadowy, marginal or indefinite area.
And, for a few days, that's right about where Petra and her family lived.
But Kim Anderson now sees her daughter emerging from that existence as she heads to rehab.
"I have hope at the very least she will now function like she did before, and I believe she will," Kim Anderson said. "I expect great things from her."