Now that the jury is in the courtroom, Holmes casts a different persona: He appears scholarly with a straight face, no small talk, well-groomed. He wears glasses, short brown hair, a trimmed beard and button-down shirts.
As the jury heard its third week of testimony this past week, here are five things that were revealed by witnesses, all on behalf of the prosecution:
1. Inside Holmes' apartment: Rigged to explode
Picture this: Pickle jars filled with napalm and bullets linked together with strands of different kinds of fuses. Plastic soda bottles filled with gasoline. A thermos containing glycerin propped to dump into a frying pan holding black potassium permanganate powder.
A mad scientist's chemistry lab?
It was the work allegedly by a doctoral neuroscience student who knew exactly what he was doing, an explosives expert testified.
"Nothing was random," said FBI Special Agent Christopher Rigopoulos, an international post-blast investigator who was consulted to "render-safe" Holmes' apartment and was part of the evidence collection team.
Evidence included a host of chemicals and equipment purchased from a local store, The Science Company, on July 14, 2012, just days before the mass shooting.
On that day, Holmes was sporting his red-orange hair, resembling the Batman villain Joker. He kept the hair color on the night of the shooting, and it became his iconic image after his arrest.
Sales associate Dylan Bieniulis remembered Holmes' fiery hair.
"No one else was in the store," said Bieniulis. "He got a few more of the metal electrodes than a normal person would. He basically grabbed all that was on the shelf."
Other than that, Holmes "kept to himself" and used a MasterCard to pay for $175.69 worth of goods, the clerk testified.
Back at Holmes' apartment, it must have taken hours for him to methodically set up all the incendiary and explosive devices "that would have functioned as designed," said Rigopoulos.
Rigopoulos described how the different chemicals, if combined, would generate tremendous heat, and set off a chain reaction, igniting fuses and triggering several fires fueled by gasoline and propane.
Eventually, the fires would reach smokeless powder-filled plastic spheres, triggering numerous explosions, Rigopoulos said.
All it would take, he testified, was for someone to trip a fishing line attached to the apartment front door -- or to manipulate a remote control left beside a toy car next to a dumpster outside the apartment building.
2. Inside the classroom with Holmes
Those who spent time with Holmes as a PhD student agree he was quiet and socially awkward, but that he never appeared "detached from reality." Holmes attended the University of Colorado Anschutz Medical Campus in Aurora,
the largest academic health center in the Rocky Mountain region.
Curt Freed, a professor of medicine, recalled Holmes' time as a student in his research laboratory at the end of February 2012 through the middle of the following May.
"He was a somewhat shy person," said Freed. He described Holmes as thoughtful and, although shy, "could go into depth on certain subjects."
Associate Professor Daniel Tollin, who gave Holmes an 'A' grade in a grant-writing course, said Holmes "was a bit reserved," but "asked good questions."
"He did outstanding. He put together a very nice proposal," testified Tollin.
However, fellow student Jessica Cummiskey, who shared a class and a lab rotation with Holmes, testified that he didn't seem very engaged, appeared to be "going through the motions, and often looked like he was "zoned out."
Holmes would avoid conversation, and rarely, if ever, made eye contact, she said.
But one thing about Holmes' eyes really stood out.
"There was more than one occasion where his pupils were completely blown out. It was shocking, stunning," Cummiskey said.
3. Holmes' dilated pupils
Holmes' enlarged pupils are likely to be brought up by his defense team as a sign of mental illness.
Defense attorneys repeatedly highlighted Holmes' dilated eyes during cross-examination.
But this line of defense is likely to be controversial. For more than 100 years, mental health experts have studied pupil dilation, but it hasn't yet proved to be a way to diagnose psychiatric conditions, according to Slate magazine
and "The Handbook of Clinical Neurology."
The defense argues Holmes suffered from schizophrenia that drove him to insanity. At least one law enforcement member testified seeing Holmes' large pupils when he was arrested.
Did Holmes' ever appear to be detached from reality, asked District Attorney George Brauchler.
"No," was the response by Cummiskey, Tollin and Freed.
They also agreed that Holmes' demeanor and behavior was consistent from the beginning of the time they knew him until he completed his last lab rotation on May 17, 2012, and gave his final presentation.
"He did an excellent job," testified Freed.
Another of his professors, William Sather, testified that he was concerned, since Holmes seemed to not be engaged in the course work or "just didn't care." Holmes went on to fail his "prelim oral exam" on July 7, 2012, which jeopardized his ability to stay in the neuroscience program.
Holmes withdrew from the University of Colorado
about that time, giving no reason, the school said.
4. Body armor and tear gas grenades
While still studying for his PhD, he allegedly began buying body armor and other equipment that would eventually be used during the movie theater attack, according to several witnesses.
The witnesses laid out a timeline in their testimony:
May 10, 2012: Holmes purchased an SG150 Gas Mask kit, which included a nuclear biological chemical filter, testified Robert Higginbotham of Approved Gas Masks store. It cost $133.74. It appeared to be the same kind of gas mask, collected and photographed after the shooting, Higginbotham said.
That same day, at 2 a.m., there was another transaction.
Thomas Platt, the owner of a website that sells firearms and military surplus, said Homes bought two 6-ounce cans of Clear Out tear gas grenades. Total: $43.79. Paid by MasterCard.
June 30, 2012: Holmes paid $858.00 for ballistic chaps (to wear around legs), testified Justin Leveck, the owner of a body-armor resale company, Blue Defense.
July 2, 2012: Holmes placed an order with Bullet Proof Body Armor HQ, according to owner Christopher Russell.
Holmes spent 903.67 on body armor: a bulletproof torso and neck protector, bulletproof arm protection and two different bulletproof groin protectors.
According to the company's tracking records, the items arrived at Holmes' Aurora apartment on July 17, 2012.
Two days later, on the night of July 19, prosecutors say Holmes gathered all of these items and drove to the Century 16 Theater.
Eighteen minutes into the movie, shortly after midnight on July 20, a gunman opens fire into the audience.
5. A crime scene worthy of war
In the movie theater, authorities later counted the "impacts" from projectiles, bullet fragments and shot pellets.
The total? 240.
The impacts pockmarked seats, walls, the floor, a staircase and even in the screen, where one mark was found.
Investigators found multiple bullet holes and other impact markings in more than 50 seats inside theater 9, including the seat bottoms.
Nicholas Carroll, a crime scene investigator with Aurora Police, listed what his team found in theater 9:
-- Six spent shotgun shells
-- 65 spent .223-caliber rifle casings
-- Five spent .40-caliber casings
-- 218 live rounds of .223-caliber rifle ammunition
-- 16 live rounds of .40-caliber handgun rounds
-- Miscellaneous shot pellets
Police also found a litany of pellets in the wall between theater 9 and theater 8, where people were also injured.
At one point, Aurora Police crime scene investigator Maria Pettolina held up a chunk of drywall from that same wall and told the jury: "Investigator Carroll and I collected an additional 20 pellets out of this area."
That testimony was the final straw for someone sitting in the victims and family members side of the courtroom gallery who whispered: "Oh, Jesus."