I've come across maybe six different ways to modify reactions, which I think I can mostly tie to different attributes rather than just relying on charisma. I do have a couple of caveats however. First, I don't equate charisma directly with "physical attractiveness" (I know most systems do, and that it's an accepted game mechanic, but it has always bugged me that physical looks are coupled with a personality trait...even though there is some degree of correlation.*) The second caveat is that I don't see every attribute directly correlating to the ability to modify the reaction of others. For instance, Constitution/health would play some degree of modification to reaction for extreme limits, i.e. physically healthy creatures would get more favorable results than obviously sickly creatures which might spread who knows what (more so in a post apocalyptic setting where the healthy creature is likely doing something right as opposed to Mr. I Bathe in the Radioactive Glow Every Day, however, with racial biases, this may actually backfire, "who does that healthy Pure Human think he is, coming in here to our mutated community and showing off his perfect body"). Third, I'm not advocating replacing player role playing as the means to modify the Reaction Results, just providing alternate methods to adjudicate the results of their attempts. (I call this the "Beauty is in the Eye of the Beholder" theory.)
For the six reaction modifier methods, I've used the most commonly seen terms and will discuss how I see them being used.
Charm/Seduce is your basic attempt at flirting/sexing up the NPC, obviously this will be based on the Charisma attribute and modified by it. Other modifiers of note would be sexual orientation of the target, which I'd probably roll on a d6 as follows: 1 - Asexual (-2), 2 - Homosexual (-2 if opposite sex), 3-5 Heterosexual (-2 if same sex), 6 - Bisexual (no modifier).
Persuade/Convince is the ability to align the NPCs views with your own through honest discourse. This would most likely be modified by the wisdom (willpower) attribute (using the bonus to saving throw table or adjustment table as applicable) rather than straight up charisma. Though, if I wanted to make a complex ruling, I would say add the bonus from wisdom to the bonus from charisma and divide the results by two.
Intimidate is the ability to physically cow the NPC into doing what is wanted. This would obviously be modified by the strength attribute (using the bonus to hit/damage table), though character level may also be considered for a modifier bonus, though there are two thoughts on this; the difference between PC and NPC level (or Hit Dice), with a penalty for being lower level than the target, and a bonus if higher level than the target, with the modifier based on the difference in level, and a straight bonus by level (say a +1 modifier at every two or three levels or something). There's also the possibility that a critically successful Intimidation attempt causes the target to flee or otherwise fail morale.
Bribe is the only method of influence not tied to an attribute or statistic, it is purely based on the amount of monetary value offered. For a simple method, I'd suggest multiplying a base value by the Hit Dice/Level of the target to determine a sufficient bribe. To determine the base value, I would suggest setting a value on each result of the Reaction Roll table (i.e. lower values for more friendly creatures and higher values for hostile creatures) and adjusting from there. For negative bribe results, I would probably have two categories, too low (reduces reaction by one result on the table), and insulting (reduces reaction by two results on the table). Bribes at half the suggested amount would be Too Low, while those of one-quarter or less would be insulting. Bribes that are more than one-half the suggested amount would simply be a fail result, or possibly might result in the target taking the bribe, but still refusing (which I'd probably put in the range of three-fourths or more below the suggested bribe level or giving a 50% chance regardless of the degree of failure of the bribe). Also, I suppose that there's the chance that the target could accept a bribe, but still betray the party/person into a trap or ambush.
Deceive/Fool/Trick/Con/Fast Talk is the ability to convincingly lie to the NPC. Before jumping to the conclusion that since thief types would naturally be adept at this and associate the modifier with the Dexterity attribute, stop and realize that a dumb character isn't going to be able to pull off a convincing lie, therefore the associated attribute would be Intelligence, using (most likely) the Additional Languages Table to adjust the reaction roll result.
Impress/Inspire/Awe is an attempt to influence the target through some means of impressive prowess. I'd probably use this as a catch-all category for using any other attribute's bonuses to modify the reaction roll, such as displaying feats of strength, dazzling displays of dexterity, engaging in an imposing example endurance, etc.
Since I haven't quite finalized which method I'd actually use for the Reaction Roll Table, I've been tinkering with a few variants. Of course, since many systems have racial reaction modifiers, I haven't touched on those either.
REACTION ROLL TABLES (Percolating)
|2d6||1d20||Simple Method||Bribe Value|
|3-5||5-8||Hostile, may attack (50% chance)||50|
|9-11||13-16||Favorable, Accepting (50% chance helpful)||5|
|2d6||1d20||Complex Method||Bribe Value|
|3-4||3-5||Hostile, may attack (50% chance)||50|
|5-6||6-8||Negative, Indifferent, Uninterested||25|
|10-11||16-18||Favorable, Accepting (50% chance helpful)||2|
*I even went and wrote up an attribute table for physical looks years ago that I use just to keep things simple: