"Said is dead"
There used to be an adage that the word 'said' should be avoided in dialogue tags.
I feel like it's become more common in the present day -- or at least outside of high school creative writing -- to acknowledge that the word 'said' is not only perfectly competent but oftentimes preferable to just about any other dialogue tag if you don't need to expound on exactly how a character is saying a line. 'Said' is considered an 'invisible word', like many articles, and a reader will oftentimes understand what's being communicated (that is, a character is speaking) and then move on without any issue.
Neither of these are quite correct.
While it is generally better to default to 'said' if you don't need to elaborate on how a line was delivered -- eg: shouted, hissed, whispered -- it's equally a bad idea to only use said as your dialogue tag. It's less visible than more pompous dialogue tags, and less likely to be misused (the English language is a beautiful monster and it's important to realize that even though two words mean the same thing they don't mean the same thing-- 'said' and 'orated' might mean the same thing but they have very different connotations on page), but you've also probably heard the advice to avoid using the same word too many times too close to itself.
The same happens with 'said'. I did have the pleasure once of reading a dialogue exchange where every dialogue was tagged with 'said' and it was a nightmare. I found myself begging for My Immortal-levels of silly dialogue tag attribution; at least that would be funny. This was staccato, unpleasantly repetitive, and made it very clear that 'said' is not invisible, it is simply more tolerable.
This can, of course, be weaponized if this sort of droning cadence is what you're going for-- there are absolutely in art, if you work with intent. But in this context I was not meant to be zoning out on the conversation.
The solution, however, is not to pull up your thesaurus and start having a character ask, reply, expound, pontificate, dialogue, murmur, opine, verbalize, and ejaculate. The solution is to incorporate action into dialogue tags.
Generally speaking, if you're having a back and forth between two characters, only need to establish up-front who is speaking when and only occasionally clarify going forward to make sure the reader isn't getting lost in a long dialogue. If there are relatively short exchanges or quick thoughts, it's best to just let the dialogue play out for a while until it feels like you may be losing the reader. There's no science on when to intercut a new 'he said/she said' tag; it depends on your style, depends on the rhythm of the conversation, depends on how distinct the two character's voice are.
But generally speaking, when two people are talking, they also aren't just talking. Somebody might be nervously playing with a pencil or drumming their fingers; their eyes are shifting to different parts of the room; they may be hesitating, pausing to think, sighing, mumbling under their breath. If you really want to get crazy they might even be doing something like pacing.
And herein lies the real secret to a dialogue exchange that keeps the reader from zoning out into a conversation or getting fatigues by repeatedly reading the word 'said'. Break up the dialogue with action tags. This gives the reader a break, helps re-ground the scene in physical space rather than reducing it to two talking heads in a void, and allows you to remind the reader of who is talking on what line without even needing a dialogue tag.
Compare:
“I thought you understood,” she said. “I did,” he said. “Then why did you do it?” she said. He said, “Because you asked me not to.”
Or:
“I thought you understood,” she expostulated ponderously. “I did,” he ejaculated. “Then why did you do it?” she queried inquisitively. “Because you asked me not to,” he articulated aloud.
To:
She bit her lip. "I thought you understood." "I did," he said. Her eyes snapped to his and held. "Then why did you do it?" He faltered. His hand clenched into a fist; his voice fell. "Because you asked me not to."
As a bonus, you're not only helping vary sentences (and thus improving reader mental engagement), you can incorporate a lot more characterization into a line just depending on what somebody is doing while delivering it.
