So you're writing dialogue, he said this, she said that. The story's emerging nicely. At some point, though, things start to feel too static, overly staged. To break up the speech rhythms you make one of your characters do something while he or she talks. Stand up, sit down, look away, brush a stray tress of hair from his eyes, bite his lip, open her mouth as if to say something, then think better of it. You know the kind of thing.
For sure, often you need do no more than reach for an off-the-peg phrase such as "he ran his hand through his hair, "she flinched", "he stood up" or "her eyes found his". Nothing wrong with these, per se. You'll find bazillions in prize-winning novels. Here's one from Hilary Mantel, perfect in its simplicity:
"Filthy." He sits down. "Weather. People. Manners. Morals."
Even in Mantel's garlanded hands, such generic tags don't amount to more than "INSERT ACTION HERE", and carry little additional energy. Most of the time, we're really writing "Rachel breathed in, and out again", or "Jeff's heart beat, then did it again, repeatedly", or "Dave moved from one place to another". They break up monotonous speech rhythms, but don't do much to deepen your readers' sense of a character's anxieties, foibles and aspirations.
But these "spare" moments actually give us an opportunity to achieve more interesting things. Take a look at this example by Alan Holinghurst. It's from his 2004 Booker prize-winning novel, The Line of Beauty:
... and Nick would say, "She's fine, she's fine," shielding his eyes from the dropping August sun, and smiling back at him with reassurance, among other unguessed emotions.
No missed opportunities here. The description of Nick's action presents a lovely, lyrical moment, far better than any variation on the desperately familiar "running his hand through his hair"/ "lowering his gaze". "Dropping sun" is unsual (and superb), but not so odd it jolts you out of the scene – unlike [INSERT ACTION]s that try too hard (think: "rubbing his cheek with his thumb, and flinching as a flap of skin halfway along the digit's length, made ragged by lifting breeze blocks earlier that day, dug into the delicate skin under his eye").
Holinghurst's [INSERT ACTION] is perfect because the action described is familiar enough that we can imagine it without too much effort, and generic enough that the description gives us space to invest Nick with attributes that mean something to us. Holinghurst hasn't over-described things, he hasn't stipulated the colour of Nick's eyes, the size of his shielding hands, etc. He even seems to pun on the pitfalls waiting for less experienced writers, who would guess at Nick's other emotions .
Here are a few more examples from outstanding writers. One is incredibly simple, one finds an unexpected word to change a familiar action into something striking, one is outrageously showy. All are beautiful:
"I saw," the colonel leers, trying to raise himself up off his perennially festering rear. "Death and damage." He falls back on the cushions.
"Very manly," says Madam Monnard, probing the cat's fur.
"Where is the funny?" my husband says, clicking the remote. "Bring me the funny."
I'll end with an example from my historical crime novel, The Cunning House (2015). Mrs Cooke, landlady of The White Swan tavern, a notorious Regency "molly" house (or gay bar, in today's parlance), is wiping down tables. As she works, she listens to the mysterious Mr Shadworth descanting on the possibility of life on other planets . As you'll see, I've interspersed simple [INSERT ACTION]s (feel free to re-use ...) with one that's more ambitiously lyrical:
“Remember, Mrs Cooke, ‘one’ is no closer to ‘zero’ than it is further from infinity.” He smiled, the perilously thin skin at the corners of his eyes becoming a mass of spiculated folds. “Infinity is all other possible numbers. It might be two, it could just as easily be two-trillion-and-two. It would certainly be a fallacy to take the sign of infinity for infinity itself.”