Kevin Winter/Getty, Charles Sykes/APBiel and Butler reportedly hit it off when filming their upcoming movie, "Playing the Field."
When Jessica Biel says "Come fly with me," what hot-blooded male could resist?
Certainly not Gerard Butler.
But after Biel invited the hot Scot to accompany her on a weekend jaunt to New York, the insider says the dynamic between them had definitely taken a turn for the warmer.
"From the beginning of the shoot, they were flirting with each other," says the source. But Biel, 29, who had recently called it quits with Justin Timberlake, "didn't want things to get weird on the set."
"They basically tried not to hook up," laughs the insider.
Early last month, however, the source says Biel had plans to hop a private plane to N.Y.C., and invited the 41-year-old Butler to accompany her for the weekend.
King Leonidas went along for the ride.
The actors' time in the Big Apple must have gone well, because, the source says, "When they returned to the set on Monday, you could see and feel the difference in their relationship." The source adds: "You got the sense that they were together."
The hard-bodied hotties weren't on location in Louisiana for very long before reports surfaced that sparks were flying between them. Us Weekly reported that at a crew member's birthday party on March 15, Butler "made a beeline for Jessica" when he arrived and "was really flirting with her."
Another witness told People.com that Biel and Butler "looked like a couple" at a cast and crew dinner that took place before filming even began.
A source also told the site that Butler, who has been linked with a lot of lovelies, including his "Bounty Hunter" co-star, Jennifer Aniston, "has a large personality, and everyone, including Jessica, gets caught up in his world."
Reps for Biel and Butler initially dismissed reports of a romance, but the story gained new life when they were spotted engaged in a more earthbound mode of travel: The "Lost" actress had her arms wrapped around the "300" star's waist as he negotiated the Pacific Coast Highway in Malibu on his Harley-Davidson.
Although he arrived solo at the soiree and spent part of the night singing along to "Pokerface," our insider says he also spent part of the evening "chatting, flirting and laughing" with "Twilight" saga actress Ashley Greene, who clearly seemed amused by him. Greene split from Joe Jonas in March.
Reps for Biel and Butler did not respond to our requests for comment by deadline.