One of the most interesting challenges that seemed to face the great Joss Whedon as he wrote the first two Avengers movies had to be:
Who to make the primary protagonist of those massive, unprecedentedly large ensemble pics?
Obviously, there was never any question that multiple characters had to have their own story arcs, and everyone had be given plenty to do. Both of those movies succeeded in that difficult balancing act. Nonetheless, it's always good to have one character to mold the A storyline around to help keep the plot and structure focused. And when you've got films packed full of movie stars and beloved comic book characters, it's not easy to know who to go with.
In the first Avengers, Robert Downey Jr. reportedly tried to convince Joss Whedon that Iron Man should be the primary protagonist, and it made perfect sense. In real life, RDJ was the biggest star in the movie (if not the world). In the Marvel Cinematic Universe, Iron Man started it all and already had the most sequels too.
But Joss Whedon went a different route, and chose a different primary protagonist to build the movie around.
Samuel L. Jackson's Nick Fury is at the crux of that movie.
Not to worry, though, for Iron RDJ, because he was placed at the center of the even more massive and character-stuffed smorgasborg that was Avengers: Age of Ultron.
That's not to say that Nick Fury had the most screentime or the most lines in the first movie (I haven't counted but I'd guess that Iron Man and Captain America both have Fury beat on that count). And I would argue that the Hulk/Bruce Banner had the most interesting/fun character arc. But the primary plot points hinge most closely on Nick Fury, and he's the one who keeps the story clipping along.
In Age of Ultron, it's not really even close. Everyone has decent storylines and arcs, but Tony Stark drives the thing, with a compelling arc to boot, and I suspect that he has the most lines and screen time too.
Along comes Avengers: Infinity War, and it laughs at the feeble efforts of the previous two movies. Joss Whedon, perhaps wisely, decided that he'd played with enough characters for a while, and decided to sit this one out. In the meantime, the Russo Brothers inherited around a dozen characters that could feasibly be set as their primary protagonist.
So who did they pick? Iron Man (since he worked so well on the last one)? Thor (who can tie together Earth and space)? Star Lord (played by Chris Pratt who is now arguably the biggest star in the world)? Black Panther (Marvel's newest MVP still riding high from his grand slam of a movie)?
Drum roll please.
That's right. In a movie stuffed with the most impressive collection of heroes from every corner of the earth and the cosmos, the Russo Brothers decided to make their primary protagonist the biggest villain in the Marvel universe.
(Beyond this point, I think a SPOILER ALERT warning is in order - although let's be honest, everyone in the world has already seen this movie).
Thanos begins and ends the movie. In the first minutes, he tangles with Thor and the Hulk for control of the Tesseract, and then in the final moments, Thanos winds things down before the credits roll by basking in the sun of some cozy, unknown world. He's the character with the clearest proactive goal - even if it is gathering the Infinity Stones to wipe out half of all life in the universe. He's the one who has to face incredible odds - I mean, I get that he's super strong, but he's still facing off against somewhere around 20 heroes. He's the one who faces the biggest moment of crisis as he has to choose to sacrifice the only person he's ever loved so he can achieve his goal. At the end of the movie, all in all, Thanos is the character who overcomes the impossible odds and achieves his goal.
Check, check, check. Thanos fills every box that you need for a primary protagonist.
Like I said before, that's not to say that there aren't plenty of other character arcs happening. Thor has easily the second biggest plot with an assist from Rocket and Groot (and a giant dwarf, Peter Dinklage). Iron Man/Dr. Strange/Spiderman have a meaty subplot too (with Star Lord eventually joining in) that's both action-packed and emotional.
However, with all of those characters running around on earth and beyond, the Russo Brothers along with their duo of ace writers, Christopher Markus and Stephen McFeely, wisely chose to use the ostensible villain to anchor the film as their protagonist. Thanos's storyline forms the spine of the film which can then be filled in and layered by the subplots and arcs of a cast so full of superstars that it would've made How the West was Won feel inadequate.
Well played, Russos and Markus and McFeely. Well played.
Now the question is: Who'll be the primary protagonist in the Avengers 4?
My money's on Captain America. But these movies (and their writing choices) are full of surprises!