I’ve combined the “social” skills (Bluff, Diplomacy, Intimidate) into one. A given PC rarely uses more than one of them anyways, and this method allows martials to be adept at both feinting and intimidation in combat. Importantly, this version has explicit alignment references (death threats are illegal in real life, for example, so good creatures should be using them sparingly if at all).

Interaction (Cha)
You attempt to influence another creature with negotiations, lies, or threats. Your Interaction check is opposed by your target's Sense Motive check or by their modified level check (1d20 + their Challenge Rating + their Wisdom modifier), whichever is most advantageous for them.

The DM assigns a modifier anywhere between -5 and +20 to your target's check, based on how well your target knows you, how risky (or rewarding) your demands are for your target, how believable your claims are, and so forth.

On a success, your target agrees to a single request you make (within reason), truthfully answers a question you have, or is similarly pliable.

You may attempt to influence a group of creatures, though the group uses the best check result from those rolled by its individual members.

'Interaction' actually encompasses three different types of checks, as described below

You lie to or manipulate a creature - for example, to convince a jailor imprisoning your allies that you are her relief shift.

Note that a creature you have bluffed in the past will most likely disbelieve you in future (and warn nearby creatures of your duplicity).

You may also bluff in combat as part of a feint.

Bluffing (but not necessarily feinting) is generally considered a chaotic act (and sometimes an evil act as well).

You attempt to persuade a creature to see things your way - for example, to secure a mutually beneficial trade deal.

Diplomacy is generally considered a good act (especially when more expedient but less savoury methods are available).

You attempt to break a creature's spirit with threats (overt or implied) - for example, threatening to slay a creature if it doesn't lead you to its treasure.

An intimidated creature is shaken for as long as it remains in your presence. Note that such a creature will invariably attempt to flee or betray you at the first available opportunity.

You may also attempt to intimidate creatures in combat. On a success, such creatures are shaken for the encounter (and may surrender, if they think defeat is inevitable).

Intimidation is generally considered an evil act (though not necessarily in combat, paradoxically).

Influencing a creature generally takes at least a full minute or longer (especially if you are attempting to influence a group of creatures). A rushed check may be made as full-round action, with a -10 penalty on the check.

Making an interaction check to intimidate or feint creatures in combat is a free action, but you may only attempt one such check per target per encounter.

Try Again
Optional. Repeated interaction checks are risky, even for successful initial checks. The DM may place a cumulative penalty as diplomatic requests become overbearing, bluffs collapse under their own weight, or intimidated creatures decide it is better to die free than live in fear.

If you are chaotic-aligned, you gain a +2 circumstance bonus on Interaction checks made to bluff creatures.

If you are evil-aligned, you gain a +2 circumstance bonus on Interaction checks made to intimidate creatures.

If you are good- or lawful-aligned, you gain a +2 circumstance bonus on Interaction checks made to negotiate with creatures (or a +4 bonus is you are lawful good).

Certain magical effects affect Interaction checks. For example, it is impossible to bluff in a zone of truth on a failed save, or to Intimidate a creature that is immune to fear. At the DM's option, your target's bonuses against confusion, charm, and fear effects apply against your Interaction checks made to bluff, negotiate with, or intimidate them, as appropriate.

If you have 5 or more ranks in Sense Motive, you gain a +2 synergy bonus on Interaction checks.