I doubt this ever happened in real life, so it’s hard to know what kind of success it would have. There are recorded instances of the traveling incognito, but the dramatic reveal?
But even in Hollywoodland, the notion isn’t quite as preposterous as it’s made out here.
Kings didn’t sit in castles, apart from the people. You couldn’t govern a country that way. In Britain, for example, the king traveled constantly with his entire retinue, being housed temporarily in the local noble’s castle. From there the king took care of legal business, settled claims, heard disputes, presided over fairs and feasts. He would hit a number of large communities every season, and would draw in as many of the locals and those from surrounding villages as could get away. The feasts alone would guarantee that. Traveling took up several months of the year and a king’s retinue moved slowly. So everybody along the way would have many chances to see the procession as it very slowly passed by.
In capital cities, there were many more chances. The king went out and about to all sorts of events, plays, fairs, feasts, visits, public events, gatherings, and opportunities to show off his face. Most if not all of the Hollywood reveals I remember take place in the city. It would have to. Even a king would need weeks to go out into the countryside for a spy mission. Slipping out of the castle for an evening, though, was hardly a big deal.
Getting a glimpse of the king was always a major event in the life of any commoner. The face would be remembered. A new king’s image probably would be passed around by sketches as soon as he made his first appearance.
Going two thousand years from Rome through quasi-modern Europe allows for too many variables to make generalizations work, and I’m sure some kings somewhere at some time weren’t well known. The other Hollywood cliché should be remembered here, though, the one where the king grumbles about the amount of work he has to do and longs for something fun. Being a king wasn’t about sitting in a castle and looming. It was a constant grind from morning till night, like a modern president. You’re never off duty and somebody always wants you.