This is going to be a slightly simple post, but I feel like it's rather important when it comes to creating characters that pop off the page.
Remember that post I wrote awhile back, about giving your characters details that are unique to themselves? Well, it kind of plays into this. Because you've got a name, hair and eye color, personality, and other traits that make your character distinct from the rest. Now you just have to keep reminding yourself that it doesn't stop there.
I mean, you don't just introduce them in your manuscript, give a description, and then make them a talking head. Character's don't speak for characters at a time unless they're making a speech, and even then there's going to be pauses, moments when they fix their shirt or make eye contact with somebody else. Most of the time, they're going to converse with others. Or even if they're on their own, there's going to be little quirks that only they do.
For example, if they're impatient, they're going to tap their foot on the ground, or their fingernails on the table. Their gaze might wander from person to person while they wait on news in the hospital waiting room. If somebody talks to them, might they get snappy? If they're waiting on something in particular, yes.
Like I said, your character isn't going to be a talking head; at least, not if you want them to be believable. Whether they're alone or with others, put the reader inside their heads (even if it's not from their point of view). Are they uncomfortable? Show it by the way they keep taking a breath to calm themselves down, or by the way they keep hugging their arms to their chest. Bored? Maybe they're picking at their fingernails.
Somebody walks into a room and surprises them: they're going to jump, and most likely the first words that come out of their mouth-regardless of what the other character says first-is going to be, "Oh my god, you scared me!" Then maybe they'll start cleaning up the glass of eater they spilled while the other repeats what they said.
Like I said, this is really basic: it's a continuation of remembering all of those details that you created way back before you even started your novel. The trick is to keep conveying these traits throughout their interaction with others, and as they progress through different events that keep changing their world. You don't have to tell the reader, "She was happy." Show the reader by the way she grins, or her eyes light up, or maybe she's bouncing up and down and trying not to jump.
Give them breaks in their speech when they look from one character to another, trying to gauge their reactions. Let others interrupt. Halfway up the stairs they hear shouting from down below; even if they decide to keep going up, they'll probably stop and try to listen in, or at least make a note of who's talking.
The bottom line: you can't rely on a description in the beginning of a scene to do all of the work for you. Show, don't tell.