This is a bit of filmmaking 101 that everybody already knows. So forgive me for even talking about something so basic, but like a lot of things I discuss, it's one of those things that everybody knows and yet people either forget to do it or they don't do it effectively as they could.
Basically it's the concept that how you introduce a character in a film should be crafted very carefully so that you communicate to the audience exactly what the character is about. The first time a major character appears, they should be presented in a way that reveals their personality strongly and clearly.
This is one of those things that filmmakers of yesteryear knew and accepted without question. Nowadays, filmmakers don't seem to think this is very important and so it's rare to see a good character introduction in modern films.
So why is it important? I think it's because you have to make the most out of every minute of film time you have. Film is "life with the boring parts cut out" and so every part of your film has to be interesting and make the strongest statement possible. Everything in a movie should be heightened - better, more exciting an more dramatic than things that happen in real life. That's just going to lead to a more interesting movie, period. So make a strong statement with the introductions of your character. The stronger you introduce them and state who they are to the audience in an abbreviated fashion, the quicker the story can get rolling. If the audience meets your main character and understands in 15 seconds that this guy is a guy who's afraid of commitment (or whatever his problem is) the quicker they get what the film is about and you can get started telling the story instead of wasting time in a complicated set-up to tell the audience who they are. Anytime you can get through set-up in a movie quickly you're on the right track. We've all seen films where the spent too much time setting up stuff or wasting time on "shoe leather" moving from location to location when they could have just cut from place to place.
Like many things in film, this works better if it's done in a smart and effortless way. The audience should get it on a more subconscious level than a conscious one. If they see what you are doing then the trick doesn't work. So it's better to not be too heavy-handed with this stuff. If your character is very complicated, then put the simple and strong statement over first and then add shadings to it as the story moves along.
Obviously, there is the other side of the coin: sometimes you want to lie to your audience. If you want your audience to think your main character is a good guy you can introduce him that way, and then you "sucker-punch" them later when you reveal he is a bad guy. The movie "Ransom" did this: you think Gary Sinise is a good cop before you find out that he is in cahoots with the kidnappers.
Two of my favorite displays of this technique are from animated movies. The classic one is the introduction of Cruella from "101 Dalmatians". First of all, it's a classic approach to have other characters talk about someone before they appear. Then you get what they're about before you even see them and it builds up anticipation about them. You can get information about what they're about or how others view them if that can save time so you don't have to do it after they're introduced.
So Roger sings a whole song about Cruella which tells you all you need to know about her. So once she appears in her whirlwind of energy you already don't like her and don't trust her. It colors all of her actions, and when she's actually being "nice" to Anita you see it as phony and insincere.
Also you get to see how she drives before you see her. It's always more effective to see a character's personality manifest itself somehow before you see the character, to see how they affect the world before you see them. And before you get to see her in person, Cruella comes roaring around the corner in her car, scattering pigeons along the way. And her car itself has a very definite look to it - instead of a generic looking car, it's got a very sinister look which tells you a lot about the driver.
But the best part about Cruella is how visually smart her introduction is. The first time you see her you see her silhouette through the glass of Roger and Anita's front door. And the image looks just like a spider in a web - a pretty clear visual symbol for someone who's creepy, manipulative and mistrustful. And yet it's not heavy-handed. Most people don't really notice it for what it is, even though Roger sings about her as being a spider. But it really gives you a creepy feeling about her before she appears which helps heighten the emotion of the story very effectively.
Another great intro that comes to mind is Scar in "The Lion King" - he's about to eat a mouse. Honestly, what kind of Lion would waste time eating a mouse? Only one too sickly, lazy and underhanded to go out and hunt wildebeests. Also you get that great moment where he slams his paw down on the mouse - when I saw it in the theater for the first time, it really made the audience jump. That put them all on edge and you watched his first scene with an upset, off-kilter feeling. Very effective.
Hitchcock, of course, is well known for his great character intros. One of his most famous textbook examples is from a film that's not that well-known: "Lifeboat". An analysis of the beginning of the film is like a master class in beginning a film quickly and effectively. He gets across a lot of setup in a really compressed amount of time.
WARNING: There are spoilers below. If you haven't seen "Lifeboat" in the 60 years since it was made and you don't want me to ruin it for you, don't read any further!
The opening credits of "Lifeboat" are shown over a smokestack from an ocean liner. But as soon as Hitchcock's credit is over, the camera pulls back to reveal that the smokestack is sticking out of the water. The the smokestack sinks completely from sight. Within a minute of the film's opening we are made to understand that a ship has just sunk.
Then the camera pans across the ocean surface. We see debris floating on the water and all of it is there to tell you information you need to grasp the situation.
First we see this Red Cross crate that tells you where the ship was from and where it was going: from New York to Great Britain (the film was made during WWII).
Then we see some other objects that personalize the passengers on the doomed ship and make them seem more real, make the situation seem more immediate: a checkerboard, a magazine, some wooden spoons. I think all of this stuff also is there to tell you that this was a civilian vessel and not a military one. Then we end on a corpse in a German lifejacket with the markings of a U-Boat on it, which tells you that a German U-Boat was responsible for sinking the ship and that the submarine itself was sunk in the conflict.
Then the camera pans up to see a lifeboat in the distance. We move closer to the lifeboat and see that there is a woman sitting in the boat.
Her clothes tell you a lot about her right away. She has just been on a boat that was attacked and sunk and yet she is wearing a fur coat and a giant diamond bracelet. She is leaning on a big set of fancy luggage that tells you she took great pains to save her own possessions amidst all of this devastation. She is smoking a cigarette and seems totally unfazed and removed from the destruction all around her.
She is so removed in fact, that she looks down and sees that her stocking has a run in it...and she sighs with irritation. All of this does a great job of telling you a lot about what kind of person she is with just a couple of shots and very little screen time - the movie is only about two minutes long at this point. She is clearly not the type of person that will deal well with the hardships of a lifeboat, that's for sure.
Immediately after this she hears a voice, and looks off. Another survivor is swimming towards her. She picks up a movie camera and films the guy as he approaches. At the time this film was made a movie camera was probably a pretty rare thing to be carrying around so this tells you that she is a journalist (I'm guessing it would back then anyway).
In this great framing device of the camera viewfinder, she films this guy swimming towards her. As he swims, he sees some money floating on the water. He stops swimming, picks up some of the money, and then pockets it before climbing into the boat.
What kind of person is thinking of picking up money after being on a boat that was sunk? Who would pause on their way swimming to a lifeboat to take the money? Again, that tells you a lot about this guy.
I haven't actually watched the movie in a while, but as far as I remember, there are several other great introductions of the other characters as they climb into the boat. Check it out sometime.
There is also a great introduction for the villain of the piece - the captain of the German submarine. In the middle of a conversation, one of the survivors on the boat looks over and his eyes widen.
Then they cut to what he sees - a hand on the edge of the boat. Then the other hand comes up. It may not seem all that effective here, but in the movie it's a very creepy and weird introduction. We find out later that he is murdering people in the boat when no one is looking. In the absence of any weapons in the boat, he is killing people with his hands, so playing up his hands stresses how dangerous he is. In fact, throughout the film several shots play up his hands which makes him seem very threatening and animal-like.
Anyway, I could go on and on but you get the drift. There are many more classic ones and everyone has their favorite character introductions. They're everywhere. Go and seek out some great ones for yourself!