Cairns watched McCullum as he walked past the dock; McCullum looked straight ahead.
Under questioning from Sasha Wass, QC, McCullum said Cairns was one of New Zealand's greatest cricketers and an ambassador for the game.
"Growing up watching New Zealand cricket, Chris Cairns was very much a superstar of that team and certainly one of my idols."
McCullum and Cairns became friends, as did their families.
Asked to describe their relationship, McCullum said: "A friend who I looked up to."
Several days before the beginning of the Indian Premier League (IPL) in 2008, McCullum said, he was having a drink with Australian cricket captain Ricky Ponting in a hotel bar.
Both were playing for the Kolkata Knight Riders.
McCullum said Cairns called him to say he was in Kolkata and had a "business proposal" for him.
They met alone in Cairns' hotel suite and shared a bottle of red wine, then ordered a curry.
There was a bit of small talk about the IPL tournament and then McCullum said Cairns asked if he knew anything about spot fixing.
"I said no. He said he would explain it to me."
McCullum said Cairns pulled out a piece of paper and pen to explain how spot fixing worked.
Ms Wass asked what McCullum thought Cairns was asking him to do.
He replied: "To underperform."
He continued: "I was shocked. I sort of thought he may have been joking. But I quickly became aware he wasn't joking ... when he kept talking in quite a relaxed nature about it.
"He said all the big boys were doing it, that I was the sort of player and personality who would take it on.
"He said he couldn't ask Dan Vettori and Jake Oram. [They] didn't have the balls to do it."
McCullum said Cairns told him that Lou Vincent and Daryl Tuffey were "working for him".
He said Cairns offered between $US70,000 and $US180,000 to underperform in games.
The 34-year-old McCullum said he asked Cairns how to get the money back to New Zealand and said Cairns said he had an associate in Dubai, who could help with property.
Asked by Ms Wass why he asked about getting money to New Zealand, McCullum said he was "inquisitive" and wondered how it happened, if someone went through with it.
Asked why he didn't report Cairns to the cricket authorities, McCullum said he "didn't want it to be true. I didn't want to rat on him."
McCullum said he told Cairns he "would think about it" and Cairns said, "And remember, this conversation never happened."
The next day, McCullum said, Cairns called him to ask whether he had changed his mind.
"I said no."
McCullum was expected to give evidence overnight about a second alleged approach in the United Kingdom, and to be accused by Cairns' legal team of lying.
- Jared Savage in London