“If bad things were happening, he never brought them to my attention,” Mr. Stanford said. “He did his job and I stayed out of his hair.”

Robert A. Scardino, a lawyer for Mr. Stanford, said in his introductory argument last week that Mr. Davis “is going to testify and admit that he is a liar and a crook, and yet these prosecutors are going to ask you to believe him.”

The defense followed that line of attack in the trial’s first week, reminding jurors during cross-examinations that Mr. Davis had often worked alone, that he had been the one in charge of finances and that he had made major executive hires.

Under questioning from the defense attorney Ali Fazel, Michelle Chambliess, a former Stanford marketing executive and government witness, acknowledged that Mr. Davis had been a powerful presence in day-to-day operations. The prosecution countered by putting on the stand Jason Green, a former Stanford Financial Group Louisiana branch manager, and asking if he ever had seen Mr. Stanford overrule Mr. Davis. “Very much so,” he said, smiling.

Mr. Green recalled how Mr. Davis had commissioned an expert to do an efficiency study when others thought she was not right for the job. Mr. Stanford berated Mr. Davis, Mr. Green said, mimicking Mr. Stanford’s jaunty Texas drawl: “He’s my best friend, but I still run the company.”

Gregg Costa, the assistant United States attorney leading the prosecution, said in his opening argument of Mr. Davis that Mr. Stanford had found someone he knew he could control: “Mr. Davis has accepted responsibility. He will give you the ultimate insider’s view.”

The prosecution concedes that Mr. Davis is facing up to 30 years of jail and hoping for leniency, but says that he will take the jury through documents and accounting records that will prove that Mr. Stanford was skimming investor money. That money, he will say, was secretly invested in real estate and other high-risk, illiquid assets, as well as personal loans and a secret Swiss bank account.

It is likely Mr. Davis will portray himself as an emotionally needy man who was easily bullied by Mr. Stanford.

Much of what Mr. Davis is expected to say has been laid out in a 2009 plea agreement in which Mr. Davis admitted to various counts of fraud and conspiracy to obstruct a Security and Exchange Commission investigation.

The fraud began as early as 1988, when Mr. Stanford owned the Guardian International Bank on the Caribbean island of Montserrat, and Mr. Davis served as his controller. Mr. Davis told prosecutors that Mr. Stanford had ordered him to make false entries into the bank’s general ledger to report false revenue and investment portfolio balances. The practice continued after the bank was transferred to Antigua and renamed the Stanford International Bank.

Over the years, as the bank sold high-interest certificates of deposit, Mr. Stanford, Mr. Davis and other executives promoted the investments as safe and secure, as they amassed assets of over $7 billion.

But by 2008, 80 percent of the money went to various risky Stanford investments. Mr. Davis said that for years, at Mr. Stanford’s request, he and others had “created false books and records.” At least $2 billion of personal loans to Mr. Stanford were concealed and disguised, Mr. Davis told prosecutors.

Mr. Stanford has pleaded not guilty to all charges.

Mr. Davis’s plea agreement was nothing if not graphic and detailed. Sometime in 2003, he said, Mr. Stanford and the two top Antiguan bank regulators had taken a “blood oath,” with Mr. Stanford pledging to provide bribes, and the officials promising not to “kill the business.” When Mr. Stanford needed money to pay the bribes, he would instruct Mr. Davis to withdraw funds from a secret numbered Swiss bank account.

When the Antiguan bank was finally running out of money in mid-2008, Mr. Davis said he, Mr. Stanford and other executives artificially inflated the bank’s assets by devising a real estate transaction in which they falsely inflated the value of a $65 million real estate transaction into a $3.2 billion asset.

Mr. Gershowitz, the University of Houston law professor, said Mr. Davis was a compelling witness.

“When you got a guy who says ‘I was in the room and I can tell you exactly what happened,’ it’s a lot easier for a jury to understand and believe than to figure it out from a mountain of paper,” he said. “It’s hard to swallow that Stanford was smart enough to make billions of dollars, but not smart enough to know what the guy down the hall was doing.”