@A.-Meowley
What you described is really about on point.
Wait a round or two. See what's going on. Pose into it. Is there a party going on? Join the party. Is there a quiet conversation? Walk into that.
Imagine what it would be like walking into that setting IRL and pose accordingly. It's not as difficult as people make it out to be and I think you're already very much on the right track. You may be overthinking it because you see other people flubbing it up so majorly and you're worrying too much that you might be, too. And I'm gonna say... I doubt you are, since you see the problematic people, too. 
Because, yeah, you've seen those people and they're a peeve. The person who is not seeing the eldritch horror and reacting accordingly. You're trying to pose your character and wondering why the ever-living-fuck Jane over there is giggling and flirting while everyone else is wigged the fuck out by the thing with twenty-eight eyes and forty tentacles.
They're not visualizing.
That's the key: visualize. (Yes even if it means imagining the many-tentacled-beast.)
Visualize. 'Okay, my buddies are standing around the kitchen looking somber... Even if my character is running in in a great mood... these are her bffs, she's probably going to scale it down and realize something is up.'
Another tip:
Even if I have very good reason to derail a scene (something happened in another scene and I have BIG PLOT NEWS to share), I will ask OOCly if it's OK before doing so.