Watch Ramon Salazar's detailed finger movements when he says "I have...absolute control."
Salazar again in the cutscene before his boss battle. He walks up a set of stairs, and it's perfectly modeled. Every footstep comes down exactly on a stairstep, and matches perfectly with no clipping. This is almost never done in game.
Leon S. Kennedy and Ada Wong's physical interactions are almost perfectly modeled, right down to the smallest contact. Again, this is almost never done in a game. Ditto for when Ashley Graham hugs Leon after her playable section.
When Leon examines the pills given to him by the dying Luis Sera, the pills roll individually in the bottle.
This game is one of the few to model natural human hip sway. It's most obvious on Ada in her tight red cheongsam, but Ashley's is also noticeable. Even Luis has it in one cutscene. Leon's is a bit concealed by the objects hanging from his belt, but even they sway naturally.
Some of this is likely due to the game engine originally being designed for PN03, which featured lots of hip swaying created with no motion capture.
Name's the Same: Ramon Salazar; the psychopathic terrorist dabbling in bioweapons, is even crazier than...Ramon Salazar; the psychopathic terrorist dabbling in bioweapons in 24.
Given there is a soldier type called Jack Krauser, it can't be a coincidence.
Shown Their Work: Luis' file which compares real life brain controlling parasites to the Las Plagas. The details on the former are impressively accurate. Given that this is a series rife with Artistic License - Biology, this is almost heartwarming to scientifically inclined players.
This is also reflected in a name of one of the weapons. While the other weapons make full use of AKA47, the Red 9 is the actual nickname for Mauser C96 pistols that were converted into the 9x19mm cartridge. The name comes from the large, ornate red number nines printed onto the grips of these converted pistols to signify the type of round they fired.
What Could Have Been: Before the developers settled on what would become RE4, they went through several prototype versions of the game, which are collectively known as Resident Evil 3.5.
Development on the first iteration of the game overlapped with that of Resident Evil 3: Nemesis (back when that game was still considered to be Resident Evil 2.1), predating Shinji Mikami's decision to switch to the Nintendo GameCube. According to an interview with 1up.com, Hideki Kamiya claims that, initially, this was to be the third RE game instead of the one that came out. It featured a private investigator named Tony Redgrave as the protagonist, and its development included several trips to Spain to study castles as the inspiration for the environments. This game was deemed to be too great a departure from the survival horror genre, and would be retooled into Devil May Cry.
Following this was the "Fog Version," whose premise involved Leon infiltrating Umbrella's European headquarters, getting infected with the Progenitor Virus and fighting fog-like creatures that may or may not have been the creations of his collapsing, virus-corrupted sanity. One of the levels was to take place on an airship, and the game would've ended with Leon dying of the virus, a move that was unpopular with the developers and one of the reasons why it was scrapped.
After that version was scrapped, Capcom made the "Hooked Man Version," which was set in a seemingly haunted mansion and had Leon fighting what appeared to be paranormal enemies, such as medieval suits of armor (which would make it into the final version), living dolls, and the titular, ghost-like "Hooked Man," a possessed-looking chap who wielded a giant hook on a chain. The quick time events and over-the-shoulder camera angles used in RE games from 4 onward first appeared here. It alternated between the "classic" fixed cameras when exploring and the OTS camera when aiming. The game was reportedly so scary that, when the trailer debuted at E3, Mikami told the audience, "Don't pee your pants." However, the game would be scrapped for being too paranormal.
The final version proposed reportedly featured zombies again, and was discarded for being too formulaic.
Ashley's hairstyle mimics that of Yorda in Ico, and her ears stick through like Yorda's as well, although Yorda's are much more prominent (believe it or not). They're both helpless females being escorted out of a castle by the player, too. However, it doesn't seem likely that Ashley is a deliberate shout out to Yorda. They're merely both the results of the same tropes. The similar hairstyles are likely due to the limitations of the hardware, as long flowing hair is computationally expensive, with the cost going up faster than the hair length. Ears sticking through the hair would mean less hair movement to model as well, and also would provide more detail.
Note that Ada uses a similar hairstyle, probably for the same reasons.
Possible shout out to The Blues Brothers when Ada takes off her sunglasses when meeting Leon after the maze. There's a bit of a reveal as Leon doesn't seem to recognize her with her glasses on. She's wearing sunglasses at night, as was lampshaded in the movie.
Franchise Call Backs
The franchise's fondness for helicopters [getting blown up] continues with Mike.
On the elevator ride in the tower in Chapter 4-4, when Leon has to keep zealots from overloading the elevator in order to rise, one of the whiteheaded zealots in red robes is marked like a zombie. A continuity nod to when a Resident Evil game wouldn't be complete without a zombie.
The opening cutscene contains footage from cutscenes in the early games.
The missile used to destroy Raccoon City, which appeared at the end of Resident Evil 3.
The oncoming zombies, which use the zombie models from the old game engines, and look both crude and more than a little out of place.
In Separate Ways, one of Ada's reports references Luis sending an email for help to a college friend he was unaware had passed on. Ada intercepted it, which got her and Albert Wesker involved. Apparently this friend was John, Ada's internal contact when spying on Umbrella in Resident Evil.
Leon tells Ashley "We're sandwiched all right" at the Cabin intro is shoutout to Barry's "You were almost a Jill sandwich" in the first Resident Evil game.
At the end of Separate Ways, Ada's helicopter flies off into the rising sun, just as the surviving members of the team do at the end of Resident Evil.
The stag's heads mounted on the wall in various places in the Castle seem a callback to the stag head in the aquarium room in the first Resident Evil.
Salazar becoming a giant plant thing that Leon must fight is a shoutout to Plant 42 in the original game. "Looks like we got to therootof the problem. - Chris.
Which itself was a shoutout to the underground tree that was the source of all the trouble in the first Alone In The Dark 1992 game.
Del Lago is a combination of the giant snake Yawn in Resident Evil and the giant crocodile in Resident Evil 2.
The dining hall with the paintings and clues is a shoutout to the previous painting puzzles in the first Resident Evil and Code: Veronica games. This really helps set the player up for the ambush.
And, of course, it references the Dining Hall in the original mansion in the first game.
The flying Novistadores around the hive are an improved remake of the bees and beehive in the first Resident Evil game.
The "dam blocking the waterfall to let the player access a secret entrance behind the waterfall" in Chapter 2-1 is the same in concept as what the player does to advance past in the barracks in the first Resident Evil game. The execution is vastly better. Perhaps a case of "this time we have the hardware to do this scene right."
Garradors are blind, dangerous, and hunt by sound. They can hear you if you run, but not if you walk. Just like lickers from Resident Evil 2.
Shooting the crates down to form a series of platforms across the water at the waterfall/dam reprises the crate pushing to form a platform across water in Resident Evil. And again in Resident Evil 2 while playing as Ada/Sherry.
And even more so when pushing the crate with Ashley into the water in Chapter 5-2, in between Iron Maidens.
The Cabin brawl in Chapter 2-2 with Louis is a much improved version of Leon travelling with Ada through the tunnels in Resident Evil 2. Ada will even kill all the zombies if Leon lets them get close enough.
The fight with U3 on the hanging rigs is a lot like the boss fight with William Birkin's dog form towards the end of Resident Evil 2. Birkin/It leaps up and down onto the tops of tall containers while Leon runs through the corridors between them.
The Separate Ways minigame, with the sequences and actions that are impossible to reconcile with the main game, is a shoutout to the same problems with the different character scenarios in the first two games.
The many rampaging trucks are a reference to the runaway truck in the opening cutscene of Resident Evil 2.
Which may have been a shoutout to the movie Duel, featuring Dennis Weaver vs. a truck.
The gondola ride is a callback to the gondola ride in Resident Evil 2.
The Butler's letter Ashley finds isn't so much a shoutout as a near identical copy of the one in Code: Veronica.
Using an eyeball to unlock an entrance is a callback to Code: Veronica, where Claire uses an artificial eye in an anatomical manikin to open the entrance to the cellar of the Prison Area.
Shooting out the first searchlight on the Military Base on the Island is a shoutout to Claire shooting out the searchlight when she first meets Steve in Code: Veronica.
Leon and Ashley counting to three then activating the switches simultaneously to unlock the door to the bulldozer/truck path is a reference to Chris and Claire counting to three and releasing the locks simultaneously to release the lock on the Linear Launcher in Code: Veronica.
Ashley uses a turnstile to open a hidden doorway to a secret area in Chapter 3-4, just as Claire does in the basement of the Prison area in Code: Veronica.
The flying robots in the boss battle with Krauser are a self reference to the little robots in Code: Veronica, where they summoned Hunters if they detected Chris.
The rewritable electronic door keys are a callback to the rewritable M-O disks used to open the cell in the secret laboratory final level in Resident Evil.
M-O disks were magneto-Optical CDs, capable of being rewritten. In 1998, this was a almost cutting edge tech. Now (2013), way behind thumb drives.
Ada throws Leon a rocket launcher, from on high, for the final boss fight. Just like Resident Evil 2.
Leon's shoulder sheathe for his knife is a callback to Claire's (non-functional) knife in a sheathe on her left shoulder in Resident Evil 2.
Leon boosts Ashley up to get into the room with the Broken Butterfly, much as he did Ada in Resident Evil 2.
And with the same opportunity to look up her skirt. Ashley normally calls Leon a pervert if he's at that angle, but not here.
Saddler's "mouth eye" is a callback to William Birkin's first form in Resident Evil 2, which had an eye embedded in upper arm muscles. Neither is a suitable place for an eye, as it would be constantly deformed by changing pressures, leading to heavily distorted vision.
When Leon and Ashley do their slow motion leap out of the stained-glass windows in a cutscene in the church, it's a callback to Steve's slow-motion leap in through a window in a cutscene in Code: Veronica.
In the cutscene in the church, Leon's "Sounds more like an alien invasion" is a reference to the movie Invasion of the Body Snatchers, where humans are replaced by identical copies under alien control.
J.J., the gatling gunner, aims by starting firing to one side of Leon, then "walking" the bullets onto his target. J.J. has an eyepatch, which means no binocular vision, which means no 3D depth perception. With Aim Down Sights, this wouldn't affect accuracy, but since the gatling gun is offset from his vision, he's forced into the makeshift procedure listed above.
The typewriter save points are a major continuity nod with the early games, but their placement frequently makes no sense. This subverts one of the usual qualities of a save point, which is that they're usually placed at some kind of traveling nexus, after/before major fights, new equipment, etc. Resident Evil 4 has typewriters in place with no strategic places at all.
When Ada faces Krauser in Separate Ways, the cutscene opens with a view of a giant muscular arm wrenching loose a steel girder to use as a weapon. This is a callback to when Birkin's first form wrenched loose a metal pipe handrail to use as a weapon vs Leon in Resident Evil 2.
Enemies that throw projectiles like hatchets, sickles, scythes, etc. only do so if they're in Leon's line of sight. This can be useful at the top of a ladder while knifing enemies as they climb up. Simply reposition Leon a little, turn 90 degrees and start knifing. Facing a wall can also help.
Note that bowgun enemies will fire at Leon no matter where he's looking. Dynamiters ditto.
In the cutscene with Ada in Ch. 5-4 on the Island, Leon temporarily succumbs to the plagas in him. The camera cuts to a close-up of his face as his eyes burn red. There's an almost identical shot of William Birkin, right down to the burning red eyes, in Resident Evil 2 as he succumbs to the virus he had just injected into himself.
Verdugo, the inhumanly quick and tough enemy with blindlingly fast melee attacks, is only weak to liquid nitrogen. And Verdugo's not all that weak even then. This is a callback to Resident Evil 3, where Nemesis is an inhumanly quick and tough enemy with blindingly fast melee attacks, and his main weakness is to freeze (liquid nitrogen) grenades. And it's not much of a weakness, merely a short slowdown, much as Verdugo is slowed for a bit.
The optional crane in Chapter 5-1 (after the first regenerators) is a callback to the short crane section in Claire's run in Code: Veronica.
When Krauser dies, he has soft tissue swell out of his chest and pulse, just like the weak spot in the first Tyrant battle in Resident Evil.