Now, the first time I read this, the detail slipped passed me (and I tend to be a very OCD reader). I think it's because we often read through the space in between the quotes more quickly than we read the quotations themselves. But even still, I had this feeling in my gut that her character wasn't happy about something, that she was closed off. That's the power of a well-used dialogue tag.
So how can we achieve this effect for our readers, even if they don't consciously realize our characterization strategy?
1) Be intentional about dialogue and its setup. The dialogue itself is very important, yes. But so is the space between what the characters say verbally. What are they saying non-verbally? Are they sitting down, looking off, rolling their eyes, fidgeting their thumbs? These things matter because they give us a particular way to interpret the dialogue.
2) Be specific. The more specific you are with dialogue tags, the better. Having a character sip of a melty Coke float gives us a lot more imagery than having a character raise their overly-glass.
3) Use characterization. Use dialogue tags as a way to express a character's unique traits and quirks. Ever notice how in real life some of your friends do the strangest things? Maybe they talk with their hands or make this funny "I don't know what you're talking about" face. Give those kinds of traits to your characters too so that you not only make them likable and unique, but you also make them realistic.
4) Subtext. Dialogue tags or beats are an excellent opportunity for subtext and for letting your reader know if he/she should trust what your characters say. So you may, for instance, have a character verbally say "I'm fine" while tapping their foot against the bottom of the table. Dialogue tags can add great layers to what is verbalized by the characters.
What other aspects of dialogue tags do you think are important or helpful to note? Do you find you enjoy writing them?