Whats that movie its a horror and the things would open there mouth up as...

[JoeK]

...wide as there head and scream at you Its a old movie havent seen it since I was a child and they would run at you and scream with there mouths open as wide as there head pls someone help
it was a long time ago its not tremors and one part of the movie were there sitting in this room or something and the guy cuts his tongue and black blood comes out and its humans that there mouths become very wide and run and scream at you to try to kill you
its not blade its not tremors not sex inthe city arr making me mad

It would be a human figure and if they seen someone there mouths would become huge and scream were they would have to cover there ears and the human things would run at these people and try to kill them

[MystMoonstruck]

Actually, this is the first film I thought of but was thinking more creature-like:
Screamers (1995)
IMDB synopsis:
On a distant mining planet ravaged by a decade of war, scientists have created the perfect weapon: a blade-wielding, self-replicating race of killing devices known as Screamers designed for one purpose only -- to hunt down and destroy all enemy lifeforms. But man's greatest weapon has continued to evolve without any human guidance, and now it has devised a new mission: to obliterate all life. Col. Hendricksson (Peter Weller) is commander of a handful of Alliance soldiers still alive on Sirius 6B. Betrayed by his own political leaders and disgusted by the atrocities of this never-ending war, Hendricksson decides he must negotiate a separate peace with the New Economic Bloc's decimated forces. But to do so, he will have to cross a treacherous wasteland where the deadliest threat comes from the very weapons he helped to create.

There is a movie trailer on YouTube, but it won't let me submit my post with the link in place! I'll try one more time since, now that I've looked at the film clip, I think this might be your movie. If you go to YouTube and type in "Screamers movie", perhaps it will bring up those clips for you.
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Actually, that effect has been used frequently, including in "The Mummy" and "The Mummy Returns". In "Critters", the creatures have big, toothy mouths, but I'm not sure they scream though they do make loud noises.

What do the "things" look like? Are they humanoid/people-like, or are they some sort of creature?

Someone above mentions "Predator". When it's unmasked, the creature did roar at Arnold Schwarzenegger's character. But, that's the only person he does it to, unless the one in "Predator 2" does it. It's been too long since I've seen the second film to recall any such scene.

Someone else mentions "Invasion of the Body Snatchers", which would be the remake since they don't do that noise thing in the original film. Their mouths do not open as wide as their heads though; they're regular people except for being pod persons.

In "Tremors II: Aftershocks", the bipedal offspring of the giant worms can sense heat, then they open their mouths and shriek loudly before racing after the prey they've located. I don't recall if the ones in "Tremors 3: Back to Perfection" do that, but some of the bipedal ones return for "Tremors 4: The Legend Begins", if I recall correctly.
NOTE: I have no idea why they changed from Roman numerals at the halfway mark.

[kristenS]

the grudge?