What makes a movie an "action" movie is its use of action - in the form of car chases, shoot outs, or fights. What makes a movie a "musical" is its use of songs/music during the film. In both movie styles, just having the particular characteristic (music or guns) does not stick the movie into a category. The movie must use the characteristic to tell the story. A film with random acts of violence that are completely irrelevant to the story will never be considered an action film, because, if the violence is not the main film narrative, some other characteristic is (such as comedy), and that is what will define which genre the movie can be classified under. A film with music in the background is, similarly, not a musical unless the music is the manner in which the movie tells its story. But it is not the fact that they use it (action, music) to tell the story, but how they use it to tell the story which makes these two genres similar.
For both musicals and action films, the plot is more contained in the music or action sequences (the "characteristic sequences", or char sequences) than it is in the non-music or non-action sequences (non-char sequences). The non-char sequences offer transitions from one char sequence to another, but are not necessary in and of themselves to understand the overview of the movie. In this way, the char sequences punctuate the plot by reasserting all the relevant information we the audience have learned in the last non-char sequence. For an example, consider the "Never Gonna Dance" number from "Swing Time", and the "Robocop on patrol" sequence from "Robocop". "Never Gonna Dance" is the song/dance between Ginger Rogers (Penny) and Fred Astaire (Lucky), occurring after Lucky's fianc‚ has shown up in New York looking for him; because of this, Penny decides to accept Romero's offer of marriage (she and Lucky were in love with each other, but Lucky has a fianc‚). During "Never Gonna Dance", Penny and Lucky start out being separated (physically - she is halfway up a flight of stairs, and he is on the lower level), but they come together and dance. At the end of the dance, however, she spins out of the room, and Lucky is alone. This directly correlates to their first meeting (on the street in New York, before they knew each other), their gradual falling in love, and their current separation (her engagement to Romero equals her leaving the room without Lucky). In the sequence I have called "Robocop on patrol", Robocop, a fallen police officer put back together using robotic parts in the hopes of providing the city with a more durable police force, goes on his first patrol. He is a superior officer because he is bulletproof, immensely strong, fast and smart. We follow him on his patrol, watch him thwart crime (most often by killing the perpetrator), and finally his eventual discovery of the main antagonist. In this char sequence, Robocop's function, as defined in the non-char sequence previous, is made clear, even without the previous explanation of why they are building him and what he will be able to do. The non-char sequence tells us, through dialog, "this is what he will do". The char sequence shows us, through action, exactly what he can do - we didn't need the earlier explanation in order to understand, after the char sequence, the who, what, and why of Robocop, but it reasserts that information to us.
A film, much like a novel, has distinct narrative points, such as the introduction or the climax. Musicals and action films use char sequences to tell the film's story at these points. In "Swing Time", the introduction takes place in the number "Pick Yourself Up", where Lucky (Astaire) and Penny (Rogers) dance for the first time, having met on the street not long before. "Robocop" uses a long chase scene/shoot-out, culminating in the death of the main character at the hands of criminals, to introduce the main character to the audience, and to explain why he becomes the way we will see him later.
In both films, the plot is then expanded by the next few char sequences. In "Swing Time", Lucky falls in love with Penny, Penny starts to fall in love with Lucky, Penny finds out about Lucky's fianc‚ Margaret, who Lucky really doesn't love anymore by that point, and Penny decides to marry Romero. Each of these plot twists is punctuated with a musical number and/or dance - "The Way You Look Tonight" emphasizes Lucky's growing interest in Penny; "A Fine Romance" is when Lucky decides he doesn't love Margaret anymore, and when Penny first finds out about Margaret; "Never Gonna Dance" is after Penny has decided to marry Romero. Lucky entices her into one last dance, where they relive their entire relationship. The climax and resolution of the film are contained in the final reprise of "A Fine Romance", where Lucky and Penny have decided to get married (as compared to the previous number, "Never Gonna Dance", where Lucky ended up alone) and live happily ever after.
Two other numbers, "Waltz in Swing Time" and "The Bojangles of Harlem", are not as strong as plot indicators but do lead from the previous number into the successive number. "Waltz in Swing Time" lies between "The Way You Look Tonight" and "A Fine Romance". Its purpose is to give the audience some idea of what Lucky and Penny can do, when they're together. "Pick Yourself Up" introduces them to each other and to the audience, but "Waltz" shows the audience why we should believe they should be together. The beauty and grace with which they dance with each other establishes what they would be as a couple, and gives the audience a reason to side with them. "The Bojangles of Harlem" comes between "A Fine Romance" and "Never Gonna Dance", and brings Margaret into the plot once again, to come between Penny and Lucky (by "Never Gonna Dance", Penny has left Lucky for Romero, because of the conflict between Lucky, Penny, and Margaret). In just following the numbers, the audience gets a fairly decent picture of what is happening through the film. Moreover, it is only during the numbers that the main plot changes occur - Penny finds out about Lucky's fianc‚ during "A Fine Romance", Lucky falls in love with Penny during "Waltz in Swing Time" and "The Way You Look Tonight", and Margaret shows up in New York during "Bojangles of Harlem".
"Robocop" continues on from the introduction of the human police officer into the introduction of the robotic version in a sequence which shows him (Robocop) on patrol in the city. This sequence, which I call "Robocop on patrol", shows the audience his abilities and gives them reason to trust him and believe in him. Much like "Waltz in Swing Time" makes the audience believe that Lucky and Penny should be the ones to get married, no matter how unlikely it seems to happen, the "Robocop on patrol" sequence puts the audience on his side near the beginning of the film, so that they can trust him throughout the film. This particular sequence concludes with Robocop destroying a drug factory, where he learns of the involvement of a high business official in the criminal underworld.
The film then proceeds on to Robocop's attempt to arrest the businessman involved with the criminal element. This is the main conflict of the film, the "good vs. bad" aspect, the equivalent of Lucky's choice between Penny and Margaret. The attempt is thwarted by the businessman who controls the police, and Robocop is nearly killed. This scene is equivalent to "Never Gonna Dance" in that we, the audience, fears that good will not prevail - Robocop will be destroyed, or Penny and Lucky won't ever be married. In the climactic char sequence, Robocop again confronts the businessman in the company of his (the businessman's) superiors, and eliminates him. Curiously enough, this is also the resolution, similar to the way in which "Swing Time" tied up the climax and resolution in the same char sequence. What I have described about "Robocop" in the last few sentences was gathered only from the action sequences, and yet the plot is very obvious, just from those scenes. As with "Swing Time", the plot's major changes only take place during the action scenes, the char sequences.
For both "Swing Time" and "Robocop", the non-char sequences, while adding to the richness of the narrative, do not change the plot nor the point of either film. The non-char sequences may, and often do, contain sub-plots (such as the drug dealers in "Robocop", or Lucky's gambling in "Swing Time"), but the major narrative of the film is solely contained within the action/musical sequences. The non-char sequences are not indispensable, but they do not tell the story, they merely add to it.
The motion pictures "Swing Time" and "Robocop" may be as far separated, subject-wise, as is possible. But structurally, the two films, and more generally the two genres, are very similar. Both films utilize a characteristic style to narrate the main points of the story. "Robocop" uses action sequences to focus the viewers attention to the plot; "Swing Time" uses dance and music to the same end. In both films, the action or music sequences (the char sequences) contain the vital changes in the plot, and the main narrative capability. The intermediate sequences color the story, but never change its direction. In both "Swing Time" and "Robocop", the introduction, thesis (or introduction of the conflict), climax, and resolution are all given to the audience during char sequences. Additional information is given during the non-char sequences, non-action or non-music parts, but these sequences are more of a link between char sequences than plot modifiers. Both films use char sequences not only to tell their story, but to dictate specific parts of their story - the introduction, the thesis, the climax, the resolution. The use of character sequences is what makes these two films so comparable. Though a romantic dancing duet may not compare at all with a machine-gun wielding robotic peace-keeper, the manner in which each tells their story, through cinema, is remarkably similar.