Killer7-PS2 and RE4-PS2 utilize pre-rendered cutscenes. In other words, the PS2 just displays FMV files off the disc rather than using the 3D hardware to draw the images. Specifically, Capcom recorded the cutscenes from the GameCube versions of the respective games, saved them as convincingly high-quality movies, and substituted these into the PS2 versions. So, in the case of RE4-PS2, if you got Leon and Ashley to change into their bonus costumes, the [major] cutscenes would not reflect the change since they're just static movie files.
I mentioned "precedent" because Killer7-PS2 was released in Summer '05, while RE4-PS2 didn't come out till October. A few days before RE4-PS2 was released, a Euro Capcom rep admitted that the cutscenes were simply FMVs, citing the costume example. People began digging into the PS2 disc and soon found the large movie files, confirming it. But when Killer7-PS2 came out, "all" reviews had no mention of this sort of deal, and they continued to assume the PS2 version drew just about everything the GameCube version did. Apparently the FMVs did a good job of fooling them. It also makes you wonder what else Capcom and other devs have fudged over the years.
No, I didn't buy the PS2 version (would I? HAH, NO). "the internet gave it to me."
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Have you played/beaten Killer7?