Class and Level Geekery VIII (spoilers ahoy!)

[Lothston]

Robilar's Gambit from PHB2 was proposed as another explanation for the AoO, but I disagree with it. RG says you resolve your AoO before your opponent strikes, and in the comic it's clear Roy attacks and misses before Tarquin responds, which is the order that Sidestep Charge dictates things happen. I may have missed it, but I didn't notice another explanation for how Tarquin did this.

Actually, you got that the wrong way for Robilar's Gambit: it specifically says that your AoO comes after the attack that provoked it.

Exactly. So Robilar's Gambit is perfectly applicable to the situation.

I would still definitely go with Sidestep Charge in this case though, because
1. OotS very rarely uses non-core material, much less non-SRD material, so when presented with two possible explanations it's much better to go with the SRD one.

Robilar's Gambit is from Player's Handbook II, which is part of Core material.

Sidestep Charge is listed among Psionic feats in the SRD, and originally comes from the Expanded Psionics Handbook, which is quite far away removed from Core rules. The feat itself was originally labeled as psionic, and only afterwards errataed to "general".

In the comic, AFAIK, we haven't so far seen any serious involvement of psionic rules/mechanics, apart from the token joke appearance alongside the other "obscure classes" during O-Chul's interrogation in Azure City (which included Tome of Battle characters as well, btw).

2. What Tarquin did unambiguously looks like a sidestep charge, while the visual cue for Robilar's Gambit is much more subtle (although his stance in the last panel of 850 does look like it might qualify).

What Tarquin did looks like a dodge, nothing more. He dodged the blow. It does not point to the Sidestep Charge feat any more than to a high Dex/Dodge bonus to AC. His movement to the side of Roy is shown in the second panel, and is likely a part of his trip attack with his foot.

3. Tarquin's AC has been shown to be quite high, but I seriously doubt it's not-get-hit-by-a-charging-fighter-while-using-Robilar's-Gambit high.

This is a valid point. The combined bonuses from RG and charging do make up a hefty +6 to hit for Roy. However, this isn't anything that can't be mitigated by a, for instance, high-level item with a large AC bonus. So it's circumstantial evidence at best.

Anyway, if Tarquin's build has a boatload of Fighter levels (which is obviously the case IMO), then there's no reason he couldn't take both Robilar's Gambit and Sidestep Charge (considering he spent a feat on something as useless as Snatch Arrow).

But at the very least it has to be one or the other, so I believe we could list it on the first page as "Robilar's Gambit or Sidestep Charge".

Speaking of best-looking explanation, what's your opinion on the Arrow Snatching? I initially marked it as the feat and its prerequisites, but then my attention was drawn to the magic item. Is that enough to withdraw the feat entry, or would you say it's better to assume the feat until the magic item can be proven?

Personally, I would loathe to waste a feat on Snatch Arrow. However, Quick Draw and Deflect Arrows are both reasonable choices for a Fighter with feats to spare, and Snatch Arrow does have the interesting bonus of giving you the ability to make an extra attack off your turn, even if it's with an improvised weapon (which can nevertheless be magically enhanced).

Also, Snatch Arrow is a very dramatic action, which could appeal to Tarquin with his fondness for the theatrical.

The Gloves of Arrow Snaring are a much worse choice, IMO. You spend an equipment slot for the ability to snatch an arrow twice per day. That's a very low number of uses for a melee fighter who by nature has a weakness against ranged attackers. Remember, the Gloves don't give any uses of the Deflect Arrows or Quick Draw feats, which are themselves quite useful.

And finally, as you said, it's just more likely to be a feat than a (weak) magic item.

TL;DR:
Robilar's Gambit or Sidestep Charge
Snatch Arrow, not Gloves of Arrow Snaring