Pop quiz: What's the biggest hit of Mel Gibson's acting career? "Braveheart"? One of the "Lethal Weapons"? No, it's "Signs."

Released 15 years ago this week, on August 2, 2002, the alien-invasion thriller not only marked Gibson's biggest blockbuster, but was also the second biggest hit for writer/director M. Night Shyamalan (after "The Sixth Sense") and the professional acting debut of Abigail Breslin.

Still, as many times as you've watched "Signs" over the years, there are still mysteries hidden in the crop circles -- from the eerie way Joaquin Phoenix landed his role to the reasons you barely get to see the aliens. So put on your tinfoil hat, check out this list, and swing away.
1. After the massive successes of "The Sixth Sense" and "Unbreakable," Shyamalan set himself a high bar for his next script. He said he wanted to write something that had "global significance" and a "universal message," but that also worked as a fun, scary "roller coaster ride."

2. Besides the real-life phenomenon of unexplained crop circles, Shyamalan's inspiration was classic sci-fi/horror movies like "The Birds," "Invasion of the Body Snatchers," and "Night of Living Dead," films where an apocalyptic menace is distilled down to a small group of people holed up in a single, remote house.
3. Initially, Shyamalan envisioned his protagonist, Graham Hess, as an older man, and he approached both Clint Eastwood and Paul Newman to star. Both turned him down, but Mel Gibson found the script a page-turner and happily accepted the role.

4.Mark Ruffalo was cast as Graham's brother, Merrill. But shortly before the movie went into production, he dreamed he contracted a brain tumor during the shoot. The next day, he went to the doctor and found he really did have a brain tumor, forcing him to drop out of the film and be replaced by Phoenix. The tumor proved benign, but it cost him his hearing in his left ear and nearly paralyzed his face. It took about a year before he had recovered enough to act again.
5. In real life, crop circles usually appear in wheat fields, but Shyamalan thought corn would be scarier because it's much taller and harder to flatten.

6. Shyamalan, who likes to film his movies in and around his hometown of Philadelphia, needed to find a Pennsylvania location where he could plant 40 acres of corn and have it grow tall in time for the shoot. He found it on the grounds of Delaware Valley College, an agricultural school that was so impressed with the irrigation method the production used (involving reclaimed water) that it added it to the curriculum.
7. At Abigail Breslin's audition, there was a dog who quickly became attached to the five-year-old and tried to leave with her. Like the character, its name was Bo. Breslin saw this as (you guessed it) a sign.

8. The besieged Victorian farmhouse seen in the film was a hollow set built for the movie. Shyamalan meant for it to represent America as a fortress; it was no coincidence that he had it painted with red, white, and blue trim.
9. The shoot started on September 12, 2001. Cast and crew held a candlelight vigil to mark the previous day's terrorist attacks on New York and Washington, then shot the film's most heartbreaking scene, when Graham talks to his dying wife for the last time.

10. The director said he cast himself as Ray Reddy, the remorseful driver responsible for the accident that killed Mrs. Hess, in order to make the film feel even more personal to himself. Gibson didn't know he'd be playing the scene opposite Shyamalan himself until just before the cameras rolled.
11. The day before Shyamalan filmed that scene, his grandfather died. He performed the mournful moment with two photographs in his pocket, one of his grandfather, and one of the first day's candlelight vigil.

12. "Signs" was composer James Newton Howard's third collaboration with Shyamalan, and they had developed such a rapport that he began writing the music even before a single frame of the film was shot, using the director's storyboards as his guide.
13. The aliens barely appear in the film. They're seen only in quick glimpses during the first 90 minutes of the film, and then revealed in full for only a minute and a half of screen time during the last half hour.

14. One reason Shyamalan kept the aliens largely off-screen was to create Hitchcock- and Spielberg-style suspense.
15. The other reason is that Shyamalan had been loath to use CGI up to that point in his career, but here, he had to add the aliens in post-production. Despite their limited screen time, the creatures proved difficult to create. Using motion capture, he had a lithe actress play the alien as graceful and stealthy, but that turned out not to look scary enough, so he replaced her with a burly male actor.

16. The alien's hand seen sticking out below the pantry door was an animatronic prop. Later, when the alien holds the unconscious Morgan (Rory Culkin) in its arms, that clawed hand was used again, but the rest of the alien's body was added in later; Culkin lay suspended in a harness whose wires were digitally erased later.
17. "Signs" cost $72 million to make. It returned $228 million in North America and a total of $408 million worldwide.

18. Phoenix and Cherry Jones (Officer Paski) went on to star in Shyamalan's next movie, 2004's "The Village," but Gibson, who began to focus on directing ("The Passion of the Christ," "Apocalypto"), wouldn't star in another movie for eight years ("Edge of Darkness").