When we were doing our deep discussions of the X-Act structure, especially in thrillers, we talked about how they were preoccupied with tension. We linked this to thrillers being plot focused. Romances, though we’ve characterized them as being more intrinsically character focused, are also preoccupied with tension. (In spicier romances, they’re, um, verytense.) Back during the initial discussion, we spent a little time talking about the difference between emergent properties, e.g. the things that happen as a natural consequence of other decisions, and conscious choices. Let’s take up that topic again, but from a different angle.
Instead of looking at the effects of a thing, let’s home in on the cause, and examine plot in thrillers, and character in romance, as structural catalysts.

What we mean here is that the needs of the plot, or the character arcs, drive craft-level decisions about the piece as a whole. This is why we wind up with structures that consistently look like Freytag’s pyramid: In both cases, the plot/character has an arc, and this naturally maps onto that structure. It’s not required that a work preoccupied with a plot or character arc have that structure, but if it doesn’t, it almost certainly results from a very deliberate choice to avoid it. If that choice is executed well, but doesn’t undermine the plot or character arc as a major element of the work, you found something very interesting.

Should you encounter a work like that, sit down and ask yourself why the author made those choices, and how they managed to navigate that execution. If this is something you want to tackle doing in your own work, you’ll need to ask yourself why you’re doing it, and how you intend to make that work. You are absolutely allowed to put a structural catalyst for one kind of structure inside a work that does something different, but there are consequences for doing that, and you need to be aware and in control of them.
Also, keep in mind that while traditional thrillers and romances very commonly have an X-Act structure, you aren’t required to use that structure in order to tell your story. You should, however, ask yourself whether what you wind up with actually fulfills the category expectations you’re working in. You might produce a brilliant, thrilling work that isn’t, strictly speaking, a thriller. That’s okay, but you’re not helping yourself, or your audience, by being in denial about it.
Identifying typical structural catalysts is a great way to get your head wrapped around a particular category or genre. This will often point you to the features common in that category with some understanding of why those are common features. This helps avoid the kind of trope copying that sets you up to produce something that superficially feels like it fits, but doesn’t satisfyingly deliver the experience readers were hoping for. It will also make it easier for you to see where opportunities for interesting subversions or pushing the category boundaries into fresh territory exist.

A good practice, whether you’re learning how to work inside a particular category, or simply want to sharpen your instincts for the consequences of certain structural choices, is to examine a particular structural catalyst and its common effects, then deliberately work against those. Time travel narratives, for example, lend themselves quite easily to non-linear structures. You can learn a lot about how plot and exposition delivery interact in a structure by examining this as a catalyst. You’ll also get a lot of mileage out of working on a piece that fully embraces this as a core element, but resists non-linearity.

Can you write a strictly linear time travel story that delivers on reader expectations for a time travel story? Even if you don’t pull it off, you’ll probably learn a lot as a result of trying.