Credit: Jon Kopaloff/FilmMagic; Splash News Online

From costars to couple?

The Change-Up's Ryan Reynolds, 34, and Olivia Wilde, 27, shared lunch at Sant Ambroeus in NYC's West Village September 16 -- but this was no ordinary outing.

"He recently told her he has a crush on her," a Wilde friend tells the new issue of Us Weekly (out now).

On the date, "she had minestrone, and they both got greens," a witness says. After, the duo went into an apartment building for two hours, then walked through Tribeca. "He briefly put his arm around her," a witness adds.

Though a source close to the costars denies their lunch date was romantic, the Wilde pal insists: "She'd go out with him again."

And how are things with the other woman in Reynolds' life, his confidante Sandra Bullock, 47? "Their chemistry's undeniable," an insider says of The Proposal costars, who shared multiple hikes and dinners this summer. "Everyone would love to see them together."