If Charm Person fails...

[Tyndmyr]

I don't think he really realises what a guard is for, and how he'd be trained in a setting where people flinging magic about is a daily occurrence.

If I feel an invasive, hostile force inside my head at the time a stranger shows up, I might very well be paranoid and cautious around him even if I don't KNOW it's him. I mean...it seems pretty reasonable.

[Yuki Akuma]

If I feel an invasive, hostile force inside my head at the time a stranger shows up, I might very well be paranoid and cautious around him even if I don't KNOW it's him. I mean...it seems pretty reasonable.

Then there's the fact that guards are trained to be paranoid and cautious around anyone they don't recognise...

[Tyndmyr]

Right. They might not actually axe you right there unless other factors are in play(ie, if you were dumb enough to cast it non-stilled/silent, from where they can see you), but their attitude is going to be pretty darned bad.

I would have no problem dropping them a step lower on the diplomacy table to represent that.

[Foryn Gilnith]

To remedy these problems, consider the spell Price of Loyalty, from Player's Guide to Eberron. It's 2nd-level, touch range, and requires a focus, but there are no signs of magic. No components. Not even a hostile force if they succeed on a will save. Same effect and duration as Charm Person.

[taltamir]

To remedy these problems, consider the spell Price of Loyalty, from Player's Guide to Eberron. It's 2nd-level, touch range, and requires a focus, but there are no signs of magic. No components. Not even a hostile force if they succeed on a will save. Same effect and duration as Charm Person.

ah, the "better spell TM".
If your spell has a drawback, just find a better spell that does the exact same thing without said drawback.