"He's an employee and a friend and Robert felt responsible for him," attorney Harland Braun said.
He said Caldwell, 46, was released from jail early Friday. He was not available for immediate comment.
Caldwell has said previously that Blake is innocent.
Blake, 68, is charged with murder, conspiracy and solicitation to murder his wife, Bonny Lee Bakley, 44, who was shot to death May 4, 2001. He remains jailed without bail.
Prosecutors won't seek the death penalty against Blake if he is convicted of murdering his wife, the district attorney's office said Thursday.
Prosecutors will instead seek a sentence of life in prison without parole, the office said.
Blake was charged Monday with fatally shooting Bakley after a dinner outing last May. Prosecutors said the actor and Caldwell plotted the slaying for months. The complicated plot evolved over time from plans to have her killed and buried in the desert to her being shot behind an Italian restaurant near Blake's home, they said.
Blake and Caldwell have both pleaded not guilty.
Besides murder, Blake is also charged with solicitation of murder, conspiracy and the special circumstance of lying in wait, which gave prosecutors the option of seeking a death sentence.
The decision was announced after a meeting of the district attorney's Special Circumstances Committee, which reviews potential capital cases.
Blake allegedly asked two stuntmen who worked with him on the 1970s TV detective show "Baretta" to kill his wife before deciding to do it himself.
Braun said stuntman Gary McLarty is expected to be the key witness against Blake at his murder trial. He said McLarty, who worked with Blake on the TV series "Baretta," is claiming that the actor tried to hire him to kill Bonny Lee Bakley.
Braun said McLarty is one of two people who say they were solicited. The other was also a stuntman from "Baretta."
McLarty had a role in another high-profile Hollywood death. He was on the helicopter that crashed on the set of "Twilight Zone: The Movie" in 1982, killing actor Vic Morrow and two children.
Braun is already trying to cast some doubt on what the stuntmen might say. He said if their claims are true, "why didn't they call the police?"
CBS News.com Legal Analyst Andrew Cohen calls Braun's announcement of the allegations by the stuntmen, "a preemptive strike by the defense designed to make public part of the prosecution's case against Blake before prosecutors do and also to immediately raise doubts about the credibility of the witnesses."
Cohen says, "I'd do this, too, if I were Blake's attorney because it takes away the element of surprise for prosecutors and it allows the credibility of the witnesses to be evaluated long before they may be called to testify against Blake."
Both men said nothing in court Monday except when asked for their pleas.
The 18 "overt" acts in the complaint allege that Blake approached two different people and solicited them to kill the mother of his baby daughter. Names of those two crucial witnesses were not immediately disclosed.
Prosecutors allege that Blake personally asked the two to kill his wife in different locations — Parker, Ariz., Three Rivers, Calif., and outside Vitello's Italian restaurant.
The complaint does not explain why prosecutors believe Blake ultimately decided to carry out the killing himself.
The complaint links Caldwell to a list of items characterized as murder implements, including two shovels, a small sledgehammer, crowbar, old rugs, duct tape, pool acid, lye and a solvent used to unclog drains.
Asked to explain, Braun said, "It's actually a list of supplies they used for pool cleaning and construction."
Blake and Caldwell were arrested last week after an investigation covering more than 900 items of evidence, more than 150 witnesses and police travel throughout the country.
Should the case go to trial, the defense is expected to focus on Bakley's past.
Blake married her after she gave birth to a child she initially said was fathered either by Blake or Christian Brando, the son of actor Marlon Brando. DNA tests showed Blake was the father of the girl, who will turn 2 in June.
Blake's attorney has suggested there could be many suspects other than the actor because Bakley ran a mail-order business soliciting money from lonely men who answered her ads in magazines and newspapers.
Police contend that Blake had the most motive because he held Bakley in "contempt" and felt "trapped" in the marriage.
On another issue, Court Commissioner Michael M. Duffey allowed TV cameras in court for the arraignment but did not rule on whether they could be present for the trial.