Here's an odd one. When you use the Fearsome Guise mesmerist trick, it's the mesmerist making the check, but the subject of the trick is the one that seems to be intimidating. (rules quoted below)
Here's where I'm getting stuck. Suppose you're an adorable little gnome mesmerist and your subject is a hulking half-orc barbarian. How do you adjudicate the size modifier for the Intimidate check? (You normally gain a +4 bonus on Intimidate checks if you are larger than your target, and a –4 penalty on Intimidate checks if you are smaller than your target.) And by the same token, would the half-orc's Intimidating racial trait apply to the check?
Fearsome Guise: A veil of illusion makes the subject appear to be someone else and then transform into a hideous monster at an opportune time. While this trick is implanted, the subject’s appearance is altered, as disguise self. The mesmerist can trigger this trick when the subject attacks a creature. The illusory appearance brief ly makes the subject appear monstrous, then ends. The mesmerist attempts an Intimidate check to demoralize the target of the triggering attack. The restrictions on attempts to demoralize a target (the target being within 30 feet and able to see and hear the creature attempting the Intimidate check) apply to the subject of the trick. This is an illusion (glamer) effect.
Fearsome Guise: A veil of illusion makes the subject appear to be someone else and then transform into a hideous monster at an opportune time. While this trick is implanted, the subject’s appearance is altered, as disguise self. The mesmerist can trigger this trick when the subject attacks a creature. The illusory appearance brief ly makes the subject appear monstrous, then ends. The mesmerist attempts an Intimidate check to demoralize the target of the triggering attack. The restrictions on attempts to demoralize a target (the target being within 30 feet and able to see and hear the creature attempting the Intimidate check) apply to the subject of the trick. This is an illusion (glamer) effect.
It is an odd one. I think RAW you simply use the Mesemeeist's Intimidate - size bonus/penalty and all - simply because the rules don't really address this.
I could see a lenient GM allowing you to use the target's size bonus because of the bolded text, but the but in brackets really spells out what they mean there.
.. would the half-orc's Intimidating racial trait apply to the check?
No you definitely wouldn't get this. RAW it's not the Half-Orc making the check so you use their bonuses, and RAI you're using an illusion to cover up the half-orc, so they wouldn't really be participating.
The other side of this is of course that a Half-Orc Mesmerist under the effects of Enlarge Person could use this on a Gnome. They would still use the Mesmerist's size bonus, which in this case - against a medium sized creature - would be +4 compared to the Gnome's -4, for a net-gain of +8. Also in this case the Half-Orc WOULD get their Intimidating racial trait (for a total of +10 compared to the Gnome).
Run it by your GM from both sides (mesmerist is larger, mesmerist is smaller) and get a ruling from them.
It is an odd one. I think RAW you simply use the Mesemeeist's Intimidate - size bonus/penalty and all - simply because the rules don't really address this.
Makes me wonder if a middle-ground call would be to always make the check without size modifiers. Like you said, it's the illusion that's scary rather than the subject creature per se.
Thanks for taking the time in any case! I'll definitely be running it by my GM.
Yeah possibly.
If I were running it I'd probably use the size modifier of the Half-orc, but everything else from the Gnome (in your original example).
No katter how you rule it there's a way to game the system, so as long as you make a ruling and stick to it this is as balanced as anything else.