That's right: it won't be a superhero [headlining] this film-it's a gtoup of bad guy 請問[ ] 中的字有什麼特別原因是要用headining 而不是headline嗎?

［翻譯］：對，那不會是一位超級英雄領銜這部電影，而是一群壞蛋。
［單字］：headline v.給……加標題，領銜主演
［文法］：It is sb / sth doing sth是固定結構，意為"某人或某物正在做某事"
［例句］：It must be the Batman fighting. 那必定蝙蝠俠在打架。
It’s the Flash running.閃電俠在跑。
It's the Kryptonite shining.氪石在閃閃發光
It won't be the Arrow shooting an arrow不可能是綠箭俠在射箭

"would not be headlining" when compared with "will not be continuing" is a future perfect continuous tense.
It means you are going to continuing waching "the headlining" of this film.
headlining=a gerund.

If someone "headline" a show, they are the main performer in it.
see p.723 "headline" Collins Cobuild English Dictionary for Advanced Learners

That's right: it won't be a superhero headlining this film. It's a group of bad guys.
這句裡的兩個it都是虛主詞，動名詞片語headlining this film是延遲的主詞(delayed subject)。

If someone heads a line or procession, they are at the front of it.
If you head a department, you are the person in charge of it.
If you are heading for a particular place, you are going towards that place.
If you head a ball in socer, you hit it with your head i order to make it go in a particular direction.
head當動詞用的解釋有上面這幾種，但都不合乎你所問的這句上下文內容。