Prologue complete! Let's cover some of the smaller points and other observations. Since this is the start of the game, there aren't any spoilers yet.
If you've seen the prologue episode of Fate/Stay Night: Unlimited Blade Works TV, then this won't technically be anything new. The changes are minute from what I can recall. It's been mostly subtle, and both versions are equally enjoyable. Maybe the anime a little more due to the lovely animation.
Personal observation of interest: the anime has moments where Rin is actually snappier when dealing with Kirei's phonecalls. It's interesting to note that in the game, Rin states repeatedly in her narration that Kirei terrifies her, so I suppose from an outsider's perspective, Rin tries to come across as stronger and more emotionally detached than how she really feels inside. This will likely be a recurring theme, but since I'm hitting up the Fate route (Saber-centric route) first, I won't get to see how things unfold for Rin until I attack the Unlimited Blade Works route later.
In the game, Rin narrates the entire prologue, and while it's neat to get in her head for once, she is really prone to rambling on about side stuff (i.e. here comes the forced exposition) at the most awkward times. For example, it would have made more sense for her to explain all these lengthy insights about Servants and magic when she first summoned Archer, but nope, she instead sidesteps a really actiony battle and tells us all about these things. It's vaguely amusing to have small clashes from the fight attempt to interrupt Rin's narration, but alas, she continues on without much pause until she's through.
So far, the prologue chapter serves as an introduction to the game's story and some members of the cast, while of course hinting at things to come. It's interesting that the prologue is shown from Rin's perspective instead of Shirou, but I suppose that because Shirou has no idea as to what he's going to get caught up in, we'd need someone to help us, the audience, understand the basics in advance.
The upside is that this prologue chapter is more engaging than most other prologues or intros when it comes to visual novels I've played. Since a lot of visual novels tend to feel more static visually, I'm admittedly not used to having more actiony bits shown, so if this keeps up, then F/SN will be a definite treat to play through. For the time being, prologue time is done, so let's move onto the larger story~!