There's a ton of good stuff in here, more than I can really read and digest without a corresponding six week intensive course.
Here's a handful of things that I try to do with the intent of bringing in other people. Can all of these things be done all the time to the extent I try to? No. Are they appropriate in all situations? Probably not. But an effort goes a long way.
Now, because this is the internet, please take the tone of the following as 'jovial and humorous'. That's how it's intended.
Acknowledge Everyone
Make an effort to make a post at as many characters that pass within your sphere as possible. This can be as simple as a "Good day, miss!" to something more directly engaging with them. Many of us were taught not to provoke our friends, but the basis of RP is provoking and responding to interactions. Not a cheery character? No problem. Emote stepping away from that guy on the street. Write a short line about giving that passing Banite/Paladin the stinkeye. Something. Anything, to let the player of the character who's just arrived know that you the player are aware of them. If your character wouldn't know someone is passing behind them, that's fine - you can throw out a line to the effect of "...engaged in furious haggling, the merchant seems unaware of the guards gathering behind her manacles in hand...
iActively [/i]Ignore People
Bear with me, this isn't as bad as it sounds and is actually pretty similar to the above.
Is there a character on the street with a sandwichboard, heralding the endtimes? Ignore them with an emote. Tell everyone in the vicinity that your character doesn't give a rip about the wild eyed halfling's prophecy (but you do!) by emoting your character's disdainful frowny faces. Give that weirdo the side eye, whisper to your friends, shield your...uh...familiar's? eyes from the strange man on the corner. Do this, and you've just actively engaged in someone's RP while your character is ignoring them. They might even pick up on this, and come over for a funny little interaction where they 'annoy' your character, but not you the player.
The added benefit of this is that it can be used as a player-to-player communication tool. If you're emoting your heart out out and people are responding and acknowledging that you're part of the scene, you can infer that at least someone's having fun with what you're doing. If you're out in the square and no one's sparing so much as a comma for you, well...maybe you aren't reading your audience well and need to adjust your approach.
Use tells
[This can also be called "communicate."
Did you just use the Active Ignore method above? Send the player a message letting them know you appreciate what they're doing. Sometimes, people can interpret the stinkeye from another character as the same thing as the stinkeye from a player. No need to blow up the person's screen in green, just a simple, "hey I see what you're doing, I appreciate it, godspeed." Keep those avenues open and be friendly with your fellow players.
Or maybe you just really liked an emote someone did, and you want to tell them. Chances are, they want to hear you liked the thing they did. Have a good, long afternoon of intensely emotional Beljuril shopping that you thoroughly enjoyed? Say something! Sometimes people don't feel included when you've tried in character, and simply expressing appreciation can help with that.
There are some players who don't appreciate these kinds of tells. Maybe it ruins their immersion. That's fair, they can use -notells to opt out.
Less Infosec
Okay! We've moved behind a locked door with maxed DC? Great. We've cast Truesight to check for stealthers? Cool. We've gone ahead and REDACTED to ensure that people can't scry on us? Fantastic, better throw down a -ward in a conspicuous to foil spies and blow out everyone's headphone speakers. What are we forgetting? Oh, yeah, no one else can play with us right now.
I get it, if you're meeting with the Commander (in the general, not any specific commander. ilu horc commander guy) to sniff out a mole in the guard, you might want to make an effort. But you should always consider leaving purposeful flaws in your security so others can participate in the intrigue. Step aside to whisper where stealthers can hear you. Let your whispers leak out to passerbies. Forget to ward against scrying sometimes. It's not like your character's going to get deleted if a nosy Sharran wizard hears your secrets. After all, you can trust those guys to only distribute secrets among the faithful.
Oh, also, maybe hire that nice young man who looks like he might say farewell with a "Dark blessings to your cycle!". He's trustworthy, right? Let him prove himself untrustworthy via interactions, and reap the consequences of his evilry and skulduggery.
Ask Permission To Beat Them
Is that halfling really grinding your half orc's gears? Beat the crap out of them! Wind up the old arm and let loose. Knock a tooth out - but maybe try emoting it first?
News flash, going to the death area is boring at best and it mechanically ends the loser's involvement in the scene. The area itself bland. You're stuck there for a minute, gotta navigate the maze, and aren't allowed to remember anything anyways, so there's really not a whole lot to do up there that's engaging.
So if you're going to beat the snot out that little jerk, try to slug him in the mouth with text first. The key element here is to speak in tentative voice. Don't Godemote breaking his nose with a punches the hin's face off his skull. Instead, throw out a with a snarl, sends a fist flying at picky mcpocket's face. This gives the player a chance to react in a satisfying way and be included even if they decide to take the punch.
If they're not playing ball with that, whip out the old hostiler and give them the best knife-goose subdualing you've got. Beyond that, well, you've tried to make some fun for them. Time for finger of death. But really, try to engage with people first. If you're at the point where you don't trust your fellow players to have fun with you, you've already lost the game.
Anyway, those are a few things I try to do in an attempt to make fun and include people. It's not one size fits all, but I find it helps.