A couple of times, I've been asked to explain how positive steadfast, flat-arc protagonists actually work in story structure (or "beat sheets," if you prefer that term). Luckily, as I mentioned in a previous post, the flat-arc protagonist story has pretty much all the same pieces as the change-arc protagonist story--many of them are just reversed.
This means, that frankly, a lot of the same moments happen, if a little differently.
Writing a steadfast protagonist is like being left-handed in a right-handed world. Same world. Different experience.
Since this can (like a lot of writing concepts) be a little difficult to "see," I'm gonna go ahead and guide you through a positive steadfast protagonist story.
Well, three in fact.
And even if you never intend to write a flat-arc story, this may still be helpful to some degree, as most stories will feature a flat-arc character, even if they are not the protagonist. (However, I wouldn't recommend getting hung up on trying to make a side flat-arc character hit all the same points as a protagonist one would.)
One of the most obvious differences in structure is that the positive, steadfast, flat-arc protagonist starts on an accurate worldview--sometimes referred to as the "truth." This is almost always, more or less, the primary theme of the story. By the end of the story, this worldview will be proven true, so the protagonist ultimately doesn't flip in his or her beliefs, making him or her "steadfast." (In contrast, a positive change character will start with an inaccurate worldview--the "lie" or "misbelief" or "flaw"--this is basically the "anti-theme"--the opposing argument to the "truth." The positive change character will change to the "truth," the accurate worldview, the true thematic statement at the end.)
However, many stories have more than one theme. Many stories have secondary themes.
Because of this, it's possible for the positive steadfast character to be steadfast in the primary theme, but be a change character in the secondary theme. A steadfast protagonist may or may not be steadfast for every theme. But by definition, they must be steadfast for the primary theme (obviously).
This is why you may see writers argue over whether a particular character has a change or flat arc, and why the same character may get categorized differently--it depends entirely on what thematic thread the person is pulling.
For example, in The Lion King, Simba is ultimately a steadfast protagonist in the primary theme--he believes in the Circle of Life at the beginning, and even though he temporarily loses sight of that, he ultimately holds true to it at the end. However, he's a change character in the secondary theme of responsibility--he starts out believing in irresponsibility but flips into embracing responsibility at the end. Because of this, different people may get in arguments about how he arcs.
Someday I will write a post specifically on secondary themes and secondary arcs. For now, I do want to illustrate how secondary themes and arcs play out for steadfast protagonists, as I feel they can be particularly important in understanding them (Simba's structure being just one example).