Mimic? Reign of Fire?
Reign of Fire it is. 
Fraggle Dance
Coming to wipe that silly smile off your planet.
Eevolution? I’ve never seen the movie (shocking since I am a fan of Mulder), but I remember that poster.
ETA: Seems like I’ve missed some choice ones. LOVE the Goonies and Brazil taglines.
I couldn’t remember the name of the movie…but I kept picturing a Smiley face with 3 eyes.
Edit: Yup now that you put a name to it I looked it up…the movie poster has the 3-eyed Smiley on it. 
That is correct!
Yay!
so,
Are You Watching Closely?
I got nothing! I thought about movies where things that were small or shrunken were involved… Hmm maybe plants? Or things that you take for granted become… deadly?
Watching watching… closely…
Honey, I shrunk the kids (and sequels)?
Innerspace?
Hmm…think harder. Who might you need/want to watch closely?
Nope. It really has little to nothing to do with size, in terms of why you’re looking closely.
It is a movie that someone mentioned here before. I didn’t check (59 pages!), so it’s even possible that someone already used another tagline from that movie. Either way, I remember it was definitely mentioned before as a guess (or the answer).
ETA: Also, it’s a more recent movie. Within the last 10 years.
District 9? (Though, I don’t remember anyone mentioning it)
Nope. And it’s not really that kind of a movie either.
I’ll give another tagline if 3 or more people say yes to a clue. (I say this because the second tagline is pretty darn easy.)
The Illusionist? The Prestige? heh heh… The Watchmen?
Yes. (Hopefully, not another rambo type movie.)
Good guesses Omra.
Children or cats might be something you need to watch closely…
Cats and Dogs?
Revenge of Kitty Galore?
Chipmunks?
Mama Duke? d:
The Manchurian Candidate? Deja Vu?
Ants?
A Bugs Life?
I thought maybe the Manchurian Candidate because you really need to watch politicians…
Next?
Signs?
The Village?
Lady in the Water?
The Happening?
Sixth Sense? (heh)
Edit: I may as well throw in Unbreakable?