It is the rare story that interests us when we don’t care about the characters in some way. And by “care,” I don’t mean “love.” What I mean by “care” is that we become interested in the characters to the point that we develop an emotional interest in seeing what happens to them. That emotion can be either positive or negative. Indeed, some of the most interesting stories have involved characters we dislike intensely and want to see fail.
To achieve this, films rely on character development. This usually involves using the first few minutes of the film to give you a glimpse of who these characters are so that you can find something about them that you can sympathize with or which appeals to you. This can be as little as having a character tell you the traits of the other character, such as a politician calling Dirty Harry a dinosaur and threatening to stop his violent ways, to spending whole scenes with characters acting out parts of their back-stories, like watching a priest visit his dying mother in a hospital for the poor.
A classic example of how this is done really well comes from Pulp Fiction. As the story of Samuel L. Jackson and John Travolta opens, they are driving to a hit. As they drive, they talk. They let you know that Travolta has just returned from Europe and they talk about an incident involving their employer and a foot massage, and they argue over whether or not the employer overstepped his bounds. This scene is brilliant. It sets up who these characters are, both in terms of personality and profession. It establishes their friendship, but also establishes the boundaries of that friendship and where they disagree on principles. That then becomes the conflict that drives their interactions throughout. It also let's you see how utterly calm and cold-blooded they are about the murderous deed they have been sent to perform. This scene also sets the mood of the film. It tells you that the film will be deliberate, it will be about side-stories and distractions, and that it will be told through quirky dialog about street-philosophy.
And what makes all of this work, i.e. what keeps this from feeling like filler or spoon-fed characterizations, is that the whole time, they are working their way toward their objective. In other words, the scene builds its tension from the fact that these two hitmen are on their way to kill someone, and we accept the irrelevant conversation because it seems incidental to what we are watching (even though it isn't). By comparison, if they were just sitting around talking and the scene ended with them then heading out to start their mission, this scene would feel like filler and would lose much of its tension and interest.
Now compare this to Evil Dud. The story begins with the characters arriving at a cabin in the woods. They are here to help break one of the female characters of her drug habit. Notice right away that they have already arrived and are now just sitting around waiting for the plot to begin. Moreover, the story of her drug use will have nothing to do with the plot itself as this story is not about her breaking a habit, it is about something completely unrelated that only starts after they finish yapping. For the next ten minutes each of the characters talks about their past dealings with her and her struggles. Unless you are a recovering drug addict, none of this will be interesting to you -- compare that Travolta and Jackson talking about McDonalds, nasty bosses, affairs, and foot massages... all very relatable. Note also that you don't have to like either Travolta or Jackson to be interested in what they are discussing, whereas you really do need to like the drug addict girl to care about her plight. This bodes poorly for Evil Dud even before the first word is spoken.
Then the real problem arises. As you watch these idiots whine about their friend for ten minutes, you feel your patience grinding to a halt. You know this isn't relevant to the story. You know they are filling time, hoping that you find something to like about their characters. This is because there is nothing else going on except them talking about their own characters. And you realize that the director could have conveyed the whole ten minute spiel in a single line of dialog during a better-written scene. Travolta and Jackson, by comparison, are steadily building tension by preparing to kill someone at any moment. And I think that is the real key here. Jackson and Travolta don't bore you because you are focused on their task and you get the feel of constant progress on the plot. By comparison, the Evil Dud crew have literally stopped their movie to chat. Had they given you this same information while playing with a Ouija Board or even done it while the characters were approaching the cabin and discussing the witchcraft history they were about to encounter, then you could have excused this. But they didn’t.
I really am thinking this is the key to how to avoid characterization feeling like filler. As I think about film after film I’ve seen, the good ones make sure that the characters are engaged in the plot as they engage in character development; the bad ones stop the plot so the characters can focus on character building.