You can pretend to be smart. Not to be social (well, kind of)
First, let me talk about a related issue. It is common for players and GM to have trouble accurately playing a character smarter than them. One reason for that is that unfortunately, becoming smarter is pretty difficult. Some would argue that if you aren't born smart, there isn't much you can do. Some would say that a good education could help, but otherwise, you screwed all the same. So you have to fake it.
Then what about social skills?
Bad news: unfortunately, there are less tricks to fake a social character. Fortunately, it is easier to learn.
Yes, YOU will have to become social. Like, really.
Yup, you will actually have to improve your own social skills. It will bring you a great return on investment, as you will learn how to play a character with good social skills, but also will be better at playing a total boor, since you will know exactly what NOT to do. It might also make your personal life better, which is quite nice.
Obviously, it is a long process, but at least it is possible, so here are some hints on what social people often tend to do. This doesn't mean that everyone doing those is the new MLK, but those are things that people deemed charismatic will often tend to do.
Talk slower
Again, a lot of people talk fast, in order to make sure people hear what they say. Unfortunately, it has the opposite effect, as by the time the audience register you started speaking, you are done already. Also, if you talk fast, you imply that what you are saying isn't worth the time.
Respect your words and what you are saying: talk slow. Make breaks in your sentences, and so on.
Politicians are really good at that. Observe them.
Speak passionately
You believe strongly in anything you say, so say it with all the passion you have. Accentuate a lot. Don't go:
"I think we should make an alliance with the druids yeah."
But instead:
"Me? Me, I see the children out there. The children. I see them fighting, fighting their friends, fighting their families, and for what?! (pause) For food?! (pause) And we all know that an alliance, a simple alliance with the druids would solve all of those issues, would bring food to the streets, would save this city's sons, and I think: why haven't we done it yet?"
Eye contact
Eye contact is really powerful, and the longer you can hold it the better. Practice as much as you can, until it becomes totally comfortable for you. You might end up staring for a while, so keep going until you can look straight into someone's eyes without looking creepy, or panicked, or whatever. You might also be surprised how other people have trouble with it.
Use your hands
When you speak, use your hands to accentuate what you say. If your hands are resting on the deck or in your pockets when you speak, you are doing something wrong.
Smile
Smile as much as you can, and get comfortable with it. Overall, the more facial expressions you can do, the better, but smiling is one of the best things, it makes everything you say more powerful.
Call People By Name.
Very powerful, makes everything you say immediately more important and personal. There is a world of difference between this:
How are you doing?
And this:
How are you doing, Samantha? (if your name is Samantha; else it's just awkward)
Other body language tips
Stand straight. Even if a character is slouching in a throne, keep your back straight and head high. Keep your arms/legs wide, to take space. If you want to have 'social advantage' over someone, avoid facing them, and instead keep a ~30 degrees angle with them. Oh, and stop fiddling with you rpen, or hair, or dice, or whatever.
Observe
As mentioned above, observe how actors and politicians speak. Start with Bill Clinton, who is often described one of the most charismatic persons alive, so much that it's considered a super-power. There are a lot of his speeches around, and analyses of what he does right. My favorite is this speech where you get a wonderful comparison between his opponent poor social roll, and Clinton's automatic success. Seriously, he just took 10.
Also look at some actors (e.g. Clooney, Gosling, Brosnan, Connery, Duchovny), see how they stand, how they speak, how they use their hands, how they do eye contact, and so on.
Finally, you can also look at this book, which despite its name is actually a pretty straightforward book, and widely recommended.
The best part: Practice!
Now that you've learned all that, use it! Fortunately, interacting with people is something we do way more often than waving a sword around, so you can find training situations everywhere. Learned how to smile to the barrista at Starbucks, practice to speak slowlier and louder when you're telling a story to your friends, learn how to use your hands when you're making a presentation at work, and so on.
And play social characters! Force yourself to pretend you're social, and trying to fake it until, as usual, you end up making it!
Bonus advice: don't mumble
People don't realize how soft they speak, and how counterproductive it is. Because they are so used to hearing their own voice, a lot of people grossly overestimate how loudly they are speaking, and can be barely audible without noticing it. If people often ask you to repeat what you said, speak a bit louder.
Best Answer
I'm playing the adventure with the same angle (Ned as a Brotherhood Spy). Here's how I'm playing it:
Backstory:
1-2 years ago: Skerrin and the other Brotherhood agents in Saltmarsh have found out that Gellan Primewater secretly does business with the Sea Princes. They start hatching a plan to reveal this to the public without bringing any attention to the Brotherhood.
some months ago: Skerrin finds out that Sanbalet's Gang (who have dealings with both Gellan Brimewater and the Sea Princes) have established their base of operations in the haunted house. He sends Ned in as a spy to infiltrate the gang.
Ned joins Sanbalet's Gang (basically in deep cover), waiting for a signal from Skerrin.
Meanwhile, the other Brotherhood agents in Saltmarsh manipulate the poacher to check out the house by mentioning the treasures that are surely to be found within.
Some days ago, the plan finally pays off. The poacher checks out the house, is scared off by Sanbalet's magic (just like the Brotherhood expected) and returns to town to tell the authorities about this (also just like the Brotherhood expected).
Skerrin manipulates Anders to hire some adventurers to vanquish the evil undead in the haunted house, then he contacts Ned (through whatever means of communication they have) that the time has come.
Ned, volunteering for watch duty on the upper floor waits for the adventurers to show up, then drops his belongings in the storage room, locks himself in the bedroom, gags himself and ties himself up (which is something he can do apparently). Then, when he hears the adventurers enter the house, he makes some noise to draw attention to himself.
Ned's fake story is the same as the one described in the book: He's a traveler on his way to Saltmarsh who was knocked out by unknown attackers in this house. But his goal is to make sure that the adventures find out about Gellan's involvement with Sanbalet and his gang. So unlike what it says in the book, Ned will not try do dissuade or hinder the plays, he will actually help them along, giving subtle hints and making sure that they find all the necessary clues (without giving away the game of course).
So he will want to tag along, but at the same time he needs to make sure that none of Sanbalet's gang see him (because they would of course recognize him and thereby tip the players off that his story about being a traveler is a lie). And the answer to how he does that is the answer to how I play him:
He acts like a coward. He refuses to leave the players because he's too afraid to continue travelling to Saltmarsh by himself. And he won't join the players in any of the fights because he doesn't know how to fight. So while the players take out everyone of Sanbalet's gang, he just stays behind and only shows his face again after the fight is over.
It's stupid, it's convoluted, but hey, it sounded to me like a lot of fun. :)