With advance tickets selling out and millions of fans causing near-riots at public cast appearances, the vampire drama Twilight is shaping up to be one of the biggest blockbusters of the year. But the real reason the movie -- based on Stephenie Meyer's best-selling book -- looks like such a success is the intense romantic chemistry between co-stars Kristen Stewart and Robert Pattinson. Kristen, 18, plays Bella Swan, a mortal teenage girl who falls for handsome vampire Edward Cullen, played by Robert, 22. "Kristen and Rob bonded from the moment they met," an insider tells Life & Style. "They got along so well that people were convinced they were secretly in love."

Sparks certainly flew from the start. "She and Rob had a lot in common, and it didn't take long for them to develop a very close relationship," reveals the insider. "They were very touchy-feely around each other and they seemed to share a lot of inside jokes." In fact, Robert was overheard proposing to Kristen on set on several occasions -- apparently he was trying to stay in character -- and he constantly recommended books to her, even buying her a copy of Doomed Love by Virgil. "I felt like an idiot just following her around," Robert recently admitted.

Though Kristen is currently dating actor Michael Angarano, with three Twilight sequels reportedly in development, Robert may have plenty of time to work his charm on Kristen!

For more inside information on Kristen and Robert's sizzling relationship, pick up the latest issue of Life & Style, on newsstands now!