There's two ways to make the loner character work in an rpg.
There are two ways to make the loner character work in an RPG
First, descriptively.
First, descriptively. ConstantlyConstantly narrate HOW you do things, - the gestures, the attitude that comes across in your actions, along with the internal monologue. (Brian Ballsun-Stanton's answer is very good about this).
Second, have small conversations instead of big ones
Second, have small conversations instead of big ones GetGet aside with another PC and have a one-on-one conversation. This is pretty much what you see happen a lot with classic loner characters in comics, movies, etc. They DO have conversations, it's just that they are very private ones. This can be more difficult for some RPGs if the group is used to being in a party at all times, instead of breaking up for short scenes as smaller groups or solo scenes.
Also, let your group know,: "My character is a loner/quiet person BUT that means we're just going to engage differently rather than not at all. It might make more sense to ask me to narrate my gestures than to ask in-character how I'm feeling, or to meet me one on one", soone."
Give everyone canin your group the chance to work with it.