You’re a step ahead, writers, if you instinctively avoid these pitfalls when you introduce characters. I've stumbled into most of them.
Showing or talking about several new characters in short order, say within a page. Even well-regarded novelists do this at times. Readers with heavy intellect might be able to peg and remember these people but the rest of us skim or read back for clues to who they are. Or close the book. If the main character goes into a room and meets five people, let them be encountered slowly and one at a time, with dialogue or action in a little scene. Of course, such scenes can be boring if they serve only to introduce these people. But if the main character meets them while engaged in his immediate goals, the introductions take on a greater purpose and the scenes can be more compelling.
Introducing with too many physical details. Details can be as dull as an info dump. For example, a story begins with the main character observing a beautiful woman. There's a sentence about her hair, one about her lips, her limbs, her supple neck, eyes, etc. Who cares? If there is no attitude embedded in the description and the reader does not yet know the main character, detail may not work. As the reader, I don't know what to think—I don’t know the point of view. The woman’s description could be introduced with attitude, revealing something about the observer (he's sleazy, lonely, repelled, whatever), but it's easier and more dependable to first let the reader know something about him. I don't know how well this succeeds, but If I can't describe a character with one great line (and usually I can't), I try to mix the details with action and filter them a few at a time.
Having one or more characters who never appear. They may be dead parents, or exes, or mentors, often part of the main character's back story, somehow influencing the current situation. While they're real in the writer's mind, the reader probably needs to see them to understand the MC’s feelings about them. I say "probably," because a great writer may find a way to make the relationship real while keeping the character offstage. But seeing is believing, right?
Having characters with no clear role in the story, or who serve the same role as another. Any character who is more than part of the setting needs to be included for a reason, either what she reveals about the main character or events, or how she affects the story. By the end of a first draft, I hope to have only essential characters with clear roles. A character with some presence in the beginning who fades away before the end without significant impact may need to be revised or written out. But throughout that first draft, the characters can be loose. For example, in my current project, the main character has received a present that makes her uncomfortable. She doesn't know who sent it but has two, maybe three suspects. I think there might be a fourth, but I'm only halfway through the first draft, and that character is still loose, so I don’t know yet. I'm enjoying the suspense.
Good techniques--things to do--are easier to practice and establish than good style, which is more dependent on artistry--word choices and unique ways of seeing. And important as they are, good techniques, like well-constructed sentences, do not guarantee that characters or story will make readers care. More about that later.