Police said Holmes bought a ticket, entered the theater, and sat in the front row. About 20 minutes into the film, he left the building through an emergency exit door, which he propped open with a plastic tablecloth holder.

<SNIP>

He then went to his car, which was parked near the exit door, changed into protective clothing, and retrieved his guns.[8][9] About 30 minutes into the film, police say, around 12:30 am,[10] he reentered the theater through the exit door. He was dressed in black and wore a gas mask, a load-bearing vest (not to be confused with a bulletproof vest), a ballistic helmet, bullet-resistant leggings, a bullet-resistant throat protector, a groin protector, and tactical gloves.[11] Initially, few in the audience considered Holmes to be a threat. According to witnesses, he appeared to be wearing a costume, like other audience members who had dressed up for the screening.
https://en.wikipedia.org/wiki/2012_Aurora_shooting#The_shooting