Within a few years of the Promised Day, General Roy Mustang was the most famous military officer in the nation of Amestris. The reasons for his notoriety had less to do with news reports of his heroism on the Day itself – reports based in part on military dispatches that were, deliberately, too obscure for the average citizen to have a good idea of what exactly had happened -- than with an opportunistic and inexplicably popular series of films.
The first installment, while making no mention of homunculi, secret government labs, the Elric Brothers, or any past connection to the Ishbalan War, framed "Colonel Mustang, Flame Alchemist" as the sole hero of the Day's events. Ill-advisedly, Roy decided to go see it. Or rather he decided to let Edward Elric, who happened to be in town, drag him to see it. Roy was suspicious of the coincidence of Ed passing through at just the right time -- Roy was suspicious of coincidence as a rule, and any situation that was Ed-adjacent came in for extra scrutiny – but he couldn't help being curious.
Captain Havoc somehow got involved in the plan (he suggested they should all go in disguise, and went off in search of suitable hats), but Roy almost slipped out of the office without mentioning it to Riza. The memory of the Promised Day involved too many things Roy and Riza didn’t talk about, now layered over all the things they had spent half their lives not talking about. Maybe he should let it be. But at the last minute, he decided to invite her, framing the question as flippantly as possible. “How often do you get to pay good money to watch someone pretend to be Roy Mustang?” he asked.
Looking up from her typewriter, Riza said, “I watch you pretend to be Roy Mustang every day. Why on earth would I pay for it?”
Roy almost offered to buy her ticket -- would that make it a date? would it be better or worse if he took the money for all of them out of petty cash and wrote it up as a "team-building exercise"? -- but then she gave him a look, and Roy realized that the import of her remark had not particularly been centered on cost.
Riza's refusal was probably the best idea any of them had all night. The only one that might have been better was when Roy said, "I'm sneaking in a flask.”
Havoc's impromptu disguise-rustling was inspired, but subterfuge turned out to be embarrassingly unnecessary. Movie-Roy was considerably more square-jawed and muscled-out (not to mention quite a bit whiter) than Actual-Roy. He not only completely disdained transmutation circles but activated his powers by mugging at the camera and saying, "Flame On!"
At least Roy was in the movie. The Elrics had presumably been eliminated due to the limited effects budget; it was a lot easier to make a plausible fireball than to mimic Ed's more intricate alchemy. Meanwhile, General Olivier Mira Armstrong – an undisputed hero of the day and one of the few human beings Roy was actually scared of – showed up as an unnamed blonde woman taken captive early in the action. Roy, Ed, and Havoc all assumed, until the closing credits, that this was supposed to be Riza (although none of them would ever say so out loud within her hearing.) Roy was jolted by the reversal, because one of his non-intoxicant-based methods for surviving the movie had consisted of allowing himself to indulge a mild sexual fantasy about the blonde actress. (It wasn’t really Riza, so he wasn’t disrespecting her.) He ended up with an extra layer of trauma just from trying to figure out who he'd been daydreaming about. Roy was mentally composing a gracious letter of apology to Olivier by the time he left the theater, although he wasn't sure how to specify what he was apologizing for.
The papers would say that, upon seeing the film, "the real Roy Mustang" threatened its producers with a lawsuit. While technically true, this phrasing implied more of a strongly-worded letter drafted by attorneys, and less of a late night phone call fueled by Edward, Havoc, and an entire flask of straight whiskey. But Roy's frequent and, as his counsel would later coach him to say, entirely metaphorical references to setting the studio lot on fire probably accomplished more than a cease and desist letter would have. In short order, the film was redubbed and recredited to change the names of all the characters. Roy was now called "Rick Hotspur".
There were two sequels.