The way I act and the way marijuana effects me really depends on what crowd of people I'm with... if I'm around some good friends or people I know pretty well, I'm laughing 99% of the time.
I love smoking with my three best friends the most... we talk the whole time, and all the stupid/random shit we say/do is hilarious. It's the exact same routine every time we get together; we just sit around outside on my friend's driveway or out on my porch and pass around a few joints, talking, laughing, making jokes about each other, and watchin' each other do stupid, embarrassing shit on accident (One time, we were trying to not wake up my friend's parents while we were walking down the stairs stoned as hell, and I thought I was on the bottom floor and took a step like normal. I wasn't. I crashed three steps down onto their tile floor. It hurt like shit, made a ton of noise, but we all burst into laughter).
There's times when someone just starts laughing out of no where, and then the rest of us join in because the other person can't stop. Soon, we're all laughing hysterically, and there's absolutely nothing we can do about it.
On the other hand, when I'm stoned in a more social setting (like a party), or when I'm with a bunch of people I don't know, I tend to be a lot more isolated, quiet, and I keep to myself. I just zone out on my thoughts. Too many people annoy me when I'm high. I like small groups of friends.
Either way, I get this weird sensation while high... I feel like I'm viewing and thinking things from "inside" my head/brain. I can't really describe it any better than that. I feel like I over-analyze things. Like, something happens or someone does/says something, and then my mind bursts into all these thoughts of why that just happened or how it happened. When I'm high around my girlfriend, she says I'm so quiet. I told her I think a lot when I'm high, and I do. I ponder everything, no matter how random or stupid it is. I wonder about serious and emotional things, such as the world, religion, and my relationships with other people. Like... deep, deep philosophical thought.
Also, when I say I feel like I'm watching things from inside my head, I mean I feel like a miniature me is sitting in a lawn chair in the middle of my head looking through my eye sockets at the rest of the world. Another way to describe it is like seeing everything around me happen through a projection screen. That's honestly the best way I can describe it.
Ever since I started doing MDMA, I've been having more "trippy" experiences with weed. One time I feltl like my mouth was caving in on itself, like the center of my mouth was a vortex. I couldn't stop sucking and chewing on my cheeks and tongue, no matter how hard I tried. I was physically unable to stop. Another time, I was outside by myself smoking and listening to my iPod in a chair, and I realized I couldn't lift my arm. I started flexing so bad trying to lift my arm, and slowly, to the music, my whole body starts flexing. I start bending my back and neck, lift my feet of the floor, flexing my arms. Imagine a dude from Dragon Ball Z going Super Saiyan. That was me. No fucking joke. I couldn't control it, but that's exactly what I felt like. I felt like I wasn't sitting down, but was supported by air. It was crazy when it was over.
Also, I love listening to dubstep when high... my mind warps to the music, and my friend says his does the exact same thing. We've both just sat in my car, stoned as shit, sitting in silence listening to dubstep, both of us trippin' out on the music. It's awesome.
There's a few other instances, but those are the three that come to mind.. weed is awesome.