It was Mr. Holmes's first public appearance in nearly eight months, and he was widely expected to plead not guilty by reason of insanity – an option that is still open to him and is still the most likely outcome.
As Holmes sat nearby, wearing a red jumpsuit and a full brown beard, his lawyers said he might not be ready to enter a plea until May or June.
"I don't think we could ethically stand before you and tell you we're ready to make a plea," defense lawyer Daniel King said.
Judge William Sylvester, meanwhile, acted annoyed.
"How am I to make an informed decision based on the limited information you've given me?" he asked, before entering the "not guilty" plea on Holmes's behalf.
Holmes faces 166 counts – mostly for murder and attempted murder – for the shooting, which killed 12 and injured 58.
In recent months, his lawyers have repeatedly emphasized a precarious mental state. Last week, they filed a motion to preserve video from a November hospital stay, in which they say that Holmes was rushed to such a facility because he needed psychiatric help and that he was held there for several days, "frequently in restraints."
His lawyers have also challenged the constitutionality of Colorado's insanity-plea laws, saying that they're too vague and that, particularly in a capital case, they could cause a defendant to self-incriminate during a psychiatric evaluation. (Prosecutors have not yet said whether they intend to seek the death penalty.)
Last week, however, Judge Sylvester ruled that, in fact, the statutes are constitutional and that prior case law provides clear definitions. He refused to make a judgment on their arguments that there may be particular contradictions in death-penalty cases, writing that "this Court has not and will not address any questions dependent on hypothetical facts and circumstances not before it at this time."
While a "not guilty by reason of insanity" plea might offer the best possibility for Holmes to escape execution, or even life in prison, it is also the most complex option. He would be immediately ordered to undergo an independent psychiatric evaluation by state medical examiners – with which he would need to cooperate, or risk losing any right to call other witnesses to his mental condition. Also, prosecutors would gain access to his health records.
Holmes could also be medicated during interviews by psychologists and forced to take a polygraph, Sylvester noted Monday. His lawyers have objected to the narcoanalytic interview, in which he could be forced to take "truth serum" to make him more compliant.
Given that those rulings just came through Monday, it's not that surprising that Holmes's lawyers wanted to wait to enter a plea, says Dan Recht, a prominent criminal defense attorney based in Denver. What is more surprising, Mr. Recht says, is that the judge forced the plea and didn't grant the delay the lawyers requested, especially given that Sylvester had just issued a written order the day before on the consequences of an insanity plea.
"It's clear to me unequivocally that the defense will plead not guilty by reason of insanity," Recht says. "They just didn't want to do it today, because that would trigger the court-ordered evaluation.... Given the judge's order yesterday, they wanted to think through what to do and how to declare their client for the inevitable evaluation."
Under Colorado law, the defense has to enter an insanity plea at the arraignment unless there is "good cause shown." Still, Recht says, it's fairly certain that whenever the lawyers do decide to change the plea – probably within the next 60 days – the judge will allow them to.
Prosecutors in the case have said that they will declare by April 1 whether to seek the death penalty.
That they will do so is also a virtual certainty, Recht says. But, he notes, it's not a decision set in stone. Declaring that they'll seek the death penalty "just preserves their right to do that," he says. "They can easily come off that anytime they want to."
The Holmes trial is currently scheduled for Aug. 5.
• Material from the Associated Press was used in this report.