The Girls Aloud singer will make a cameo appearance as a TV talent show judge in the movie, which is currently filming in the US. Cole and Diaz were both pictured on set earlier this week.
According to The Sun, the duo have spent time getting to know each other between takes.
Diaz was thought to be excited to meet Cole, and planned to invite the 28-year-old out for "dinner and cocktails".
After ending her Twitter silence yesterday, Cole used her second tweet on the micro-blogging site to wish Diaz a happy birthday, writing: "Happy birthday Cameron, you sexy beeeyyaatchh... X (sic)"
An insider told the paper that the two stars had been "getting on really well on set".
"Cameron loves her Geordie accent and keeps getting Cheryl to say certain words," the source claimed.
Cole made her return to the US just months after she was axed from the Fox version of The X Factor.
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Watch Cheryl Cole's 'Promise This' below: