The first requirement of persuasion or diplomacy is that everyone have the same goals or motives. That what is asks furthers a belief or takes nothing away from the character being solicited (solicitee; while the actor of solicitation is the solicitor; let's use the -ee for the target of the action, and -or for the actor) . This means anything that affects their core beliefs automatically fails.
Now when it is of several steps removed that's when it's possible to persuade. "Hard Sell" can lead a very high difficulty, -10 in GURPS or difficulty 20 in OPRGS. Hard sell works as long as the margin of success. Hard sell is immediate and temporary; while soft sell takes advantage of the cognitive biases that make us adapt and like on repetition. This is resolved on multiple times and can have the results causing a step back.
Persuasion, is not best in persuading people... It's psychology. If I know this person believes the world is flat, no amount of persuasion will change their mind. But, if you know where to approach you can introduce information that they are willing to accept, possibly want to accept, that will be inconsistent with their core belief by several steps removed.
Sadly the cognitive biases we have dictate how we gain information, and they have such rigid rules. Information that is conflicting with our core beliefs makes us uncomfortable. We can resolve it with time to reflect or talk about it, but when the information is accepted then the core beliefs shift ever so slightly. The less people talk about it or reflect on it, the more conflicting many of the accepted truths can become. Note that it is a luxury to be able to talk and reflect on things, in a very primitive society this is where people gather to socialize and reflect, sacred places of worship.
Cm by Cm is how we persuade, even people who say they're convinced are really not, they are just being accommodating or want you to believe they are. A character does not need a high persuade to tell people the truth; they need psychology to strategy where to approach, and diplomacy not to ruffle feathers when presenting the information.
In reflection, GURPS fast talk is persuade. If you check GURPS psychology, you can ask the gm how the persuadee will react to X data. You can triangulate an approach, and using players logic or philosophy skill find out which piece of knowledge will the person accept but will cause the person to reshuffle his core beliefs outlying elements.