There’s a pivotal scene in my western where my main character reveals why she does what she does and what made her the person she is. Nothing too complicated. Just a couple of lines of dialogue.
It took a few passes to whittle it down so it got to the point fast and in as few words as possible. I think it works quite nicely.
It’s been suggested how this was a great opportunity to apply the “show, don’t tell” rule and make it a flashback. The logic being that since it’s such an important moment, showing it, rather than just her talking about it, would have a greater impact.
I’m not so sure about that.
I don’t have a problem with flashbacks, but have always tried to avoid using them. I guess I see them almost as a cheat; possibly even lazy writing. Like you can’t weave that information into what’s happening now, so you stop the action to show it. But once you interrupt the momentum of your story, it’s not easy to get things back up to speed.
And sometimes a flashback just isn’t necessary.
Consider the scene from JAWS pictured above with Quint’s story about the Indianapolis. Should we actually see what he’s describing? Highly doubtful. Part of why that speech works so well is how it’s delivered. You can see and hear what that experience did to him. How you’re imagining it is much more terrifying than anything they could show. The speech would lose its impact if we were concentrating on the action, rather than what Quint is saying.
Sometimes just a line of dialogue or two can be just as effective, if not possibly more so, as pausing the action for a flashback. This isn’t to say you shouldn’t use it. If you think it’s the most effective way to make your point, then by all means do it.
Just make sure it’s a solid fit.
Maximum Z 
Agreed.
Yes, flashbacks tend to be a crutch, and people often say not to use them (along with voice-overs), but that’s because they’re typically misused. I would argue that flashbacks work best when they’re primarily visual, such as the flashback to Rick and Ilsa during happier times: It’s quick, and as I recall, it has no dialogue, so it gets the point across. But there are also moments where Rick recalls his past relationship with Ilsa and we focus on Bogart’s performance, with no flashback visual cluttering the experience.
With the example from your script, or Quint in Jaws, I would agree that telling rather than showing is better because we want to feel the power of the words and see the speaker’s face without a visual flashback getting in the way, as you say. Putting Quint’s speech over a visual flashback would have ruined the power of it, because we would have been focused more on what was happening.
Telling is sometimes faster or cheaper than showing. Or both. That’s the best time to do it.
Here’s the famous ultimate example of a two line verbal characterization:
INMATE ONE: “What was your childhood like?”
INMATE TWO: “Short.”
(Clint Eastwood’s character in “Escape from Alcatraz.”)