A lot of Shyamalan's films are concerned with belief, but this might be the one that is the most explicit about it since the main character is a pastor suffering from a crisis of faith. We get a really straightforward moment to make this theme clear:

"See what you have to ask yourself is what kind of person are you? Are you the kind that sees signs, that sees miracles? Or do you believe that people just get lucky? Or, look at the question this way: Is it possible that there are no coincidences?"

That's not the whole movie right there, but it's a big enough theme that you can chew on it a while and not go hungry.

I didn't remember Signs being as quirky as it is. The drama, humor, and creepier moments roll into each other with an ease that gives the story a feeling of levity or detachment even as it speaks to increasingly heavy subject matter. Shyamalan seems to direct the cast to deliver their performances with a bit of stiffness that gradually softens into more emoting and natural cadence on their dialogue by the end. Reminds me of a 1950s or 60s B-movie sci-fi vibe, or maybe elements of The Twilight Zone. It's a little awkward and adds to the aforementioned quirkiness, especially during humorous moments.

I think the shift from stiffer dialogue and acting to subtly more naturalistic performances also works as part of illustrating the resolution of the underlying struggle with grief that the family is contending with. For much of the story, it's as though they're going through the motions and lines of life without being fully present, like they're still reeling from the death of the mother/wife.

As the incident overtaking their community and beyond intensifies, they're forced to come to terms with their grief and each other. A reforging of family ties that also brings them back to the present moment and each other. This cracks and eventually breaks the masks of false normalcy they're wearing, especially so for Mel Gibson's character.