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Finn is the male lead in TFA. He is also a male lead in TLJ, but less than he was in the previous movie, NOT because of Kylo or Poe, but because of Luke. In TFA, the movie revolves around Finn until act 3. But TLJ, largely, revolves around Luke and what he represents. 
Finn is the main orchestrator and driver of the central plot in TFA. But TFA had a considerably more linear narrative structure than TLJ did. TLJ splits the Force and the Resistance plots. Neither is ālesser,ā but the one with Luke automatically has more weight.
The decision to give Luke a more central role over the new characters was partially necessitated by the fact that TFA also hung its central plot over āfinding Luke,ā and the reasons for both his and the Jediās isolation and flaws needed to be investigated and questioned.
TLJ picks up on the thread of bringing Luke back right after we see the Resistance in dire straights. With the Resistance plot being lead by Poe and Leia, and the Force plot being lead by Rey and Luke, where does that leave Finn, our male lead? Rian had to give him his own story
I do think Rian couldāve delved more into Finnās psyche, his thoughts, feelings and emotions while on Canto Bight with Rose, reflecting on his past as a stormtrooper, his experience with oppression, and his previous belief that he could still escape to freedom in the galaxy.
I think Finn mostly listening and internalizing Roseās experiences (while it makes sense to me within his arc) give the impression that heās a passenger instead of the driver, even though this is all for his development, and contribute to the impression of sidelining.
But it's absurd and disingenuous to suggest that Rian forgot about Finn, or sidelined Finn in favor of Kylo, when he had his own plot. Impressions of him being sidelined have more to do with the structure of the film, Luke, and minor flaws of expression in Finnās individual arc.
Rian saying Kylo is half the protagonist had nothing to do with Kylo being a male lead, which he isnāt. Rian meant that as a way to say that Kylo and Rey are reflections of each other, and that he set out to treat Kylo as a protagonist with the empathy and dedication required.
Meaning, that the protagonists in TLJ, an ensemble movie, are the characters who have some form of development or whose emotional ordeal is given weight: Luke, Rey, Finn, Poe, Kylo, Leia, and Rose. The antagonists like Hux, Phasma, and Snoke, do not get this treatment.
Iām tired of seeing people say that Rian forgot about or considered Kylo more important than Finn, or seeing people say that Poe or Rose took away from Finnās arc, when the truth is the focus on Luke was given priority over all the new characters, even Rey.
*straits 
Adding to this to talk for a second about screentime and to reiterate the point about Ben again. This thread is about Finn basically being āaā male lead in TLJ as opposed to THEE male lead, but heās still more of one than Kylo.
https://twitter.com/xspaceangelsx/status/1285247459225149442?s=21 https://twitter.com/xspaceangelsx/status/1285247459225149442
https://twitter.com/xspaceangelsx/status/1285247459225149442?s=21 https://twitter.com/xspaceangelsx/status/1285247459225149442
