"beats" in dialog

Just like "he said"/"she said", beats can be used to good effect as speaker attributions. A beat is an action or description in the middle of dailog in a story. But both can be used too often, and in the case of beats, beats that are too generic can get old fast.

Unique mannerisms are less likely to get old. nods and smiles have their place, but if someone is nodding/smiling/scowling after every single line, it may be too much. If no one ever has any facial expressions, that's probably not enough.
Keep in mind, if the dialogue is between just two people, you don't need an attribution after every line. You can assume that the speakers are alternating, in which case you can have 3 or 4 (short) dialogue paragraphs with no attribution and it can flow very smoothly.

To me, beats serve three main purposes:
1. attribution: lets you know who is saying what.
2. characterization: actions speak louder than words, this can betray a lie, show nervous habits, convey more subtle communication between characters, any number of other things.
3. pacing. A longer beat conveys a longer moment of time between speech.

An example of beats used for pacing:
Alice glared at Tom and slapped the countertop with her hand. "Tell me what you know."
Tom didn't look up from the dishwater. "I can't."
"You can't? That's baloney and you know it. This is important. You could save her life."
He rinsed a handful of silverware and set it in the drainer with a clatter. "It's not that simple."
"What's not simple?"

Once Alice and Tom start talking, she has no beats because she doesn't hesitate. As soon as he speaks to her, she has a response. She's very upset at Tom, and she isn't pulling her punches.
Tom, on the other hand has beats before both of his lines, and long ones at that. The beats slow down his responses, giving the impression of hesitation without actually saying "he hesitated". The second beat is longer than the first, implying a longer hesitation. His words make it clear he doesn't want to talk, and his actions support that by slowing his pace.
In this case the particular actions aren't even that important. Are clean dishes vital to the story? Probably not. He's fixating on them, using them to try to delay the conversation.

Also, a related point about point of view. To me, I want to see the story through the eyes of the character using the prose as a lens. What I mean by that is that so many things, down to scene descriptions, and in this case, beats, are opportunities to characterize.
In the case of my example dialogue, whoever is the protagonist notices Tom's actions in close detail during the argument. Let's say Alice is the protagonist. She notices when he sets the handful of silverware down because she's eager to continue the argument and she's frustrated at his hesitation. If she was just asking how his day was, she might not be scrutinizing every detail of his dishwashing. In that case I might have used different things for beats, something appropriate to the occasion.