The first few seconds to an opener are crucial to getting a viewer hooked. The way the animation matches the beginning beats to the theme song, what pops up on the screen first, all serve as a set up that determines whether or not the viewer will want to keep watching. This is true for both western and eastern animation.
The first few seconds of the opening theme to Star vs the Forces of Evil start with the eponymous heroine’s wand blinking in tandem with the beat to the music before Star literally jumps on screen as the supporting cast is introduced in quick succession, matching the bouncy beats of the theme song. This sets the tone for a quirky, energetic series and matches the pace at which the main character is prone to move at.
Opening themes are an effective way to introduce the main characters in the series, making it easier for the viewer to remember who’s who, especially if the cast is fairly large. This can be done as simply as featuring characters alongside title cards with their names tacked on them, or it can have the characters appearing for a certain amount of time, enough to at least give the viewer a good enough impression as to what that character’s personality is like. It’s an easy way to get the viewer to be more invested in a character, as we’re more likely to form an attachment to them if we can actually remember who they are.
Opening themes, particularly in anime, often provide an opportunity for animators to try out some experimental animation that may or may not be prevalent in the series proper. Sometimes, the opening theme itself may even be the only redeeming thing about the series in question (*cough* Absolute Duo *cough*), if only because of how nice the animation is (and that may have been where all of the series’ budget went). They also have an advantage over live action shows in that the animators know how these characters will look as the series progresses, whereas real life actors’ appearances cannot be predicted as they age. This is why almost all live action shows simply reuse shots from the show proper.
This season’s Mob Psycho 100 features a lot of wacky animation and you can easily tell that the animators had a lot of fun working on it. There’s floating heads tacked on to ferris wheels, muscle-bound men exercising atop mushrooms, aliens popping out of a girl’s head…There’s just so much going on, and yet it still manages to fit in snugly with the series’ narrative. As we continue to watch the show, we learn that the muscle bound men are all soft-hearted pacifists, the girl is a believer in the supernatural, and the heads spinning around the ferris wheels are all psychics who have abilities that could potentially have the world revolve around them (okay, well, one of them is a sham, but he’s definitely the most self-centered character in the series). The last few seconds in particular highlight each major arc through some clever, fluid animation that has each major character appearing alongside objects and symbols that reflect their character arc before literally ending with a bang as the protagonist appears on screen and lets his powers loose.
Much like how the first few lines in a novel are crucial to getting the reader hooked, what we see in an opening theme works the same way. Through the opening theme, we’re introduced to the characters, the setting, and if it’s feeling symbolic enough, a good chunk of the story. And be it subconsciously or otherwise, what we seen in the opening theme is what determines our viewership.