The meta-game trap is to first: confirm their belief. Fold in marginal spots against other opponents at the table. Maybe even do something to call attention to the fact that you "have something" and are folding anyway because you are so timid.
As for the tells
: players who see you as passive will start to make physical indicators that you are "their prey". The physical posturing for this is probably one of the most common and obvious tells you'll run into. They will face their torso and often times arm movements toward you. If you feel you are being "subtly intimidated" that's exactly what they are doing - sometimes consciously sometimes not.
Here's how it's exploitable: they aren't expecting you to switch gears. If they see you making folds against some players, they'll expect you to fold to them. Here you can start to play back at them. Since they see you as a passive player if you become aggressive they'll give you credit. Look for wet boards to make plays. If you have a value hand, let them try to push you around.
The hardest part is being able to call bluffs with marginal hands. If you can piece together the picture of what your villain is doing you'll catch them trying to push you out of pots. You don't HAVE to do this. You can hold Ace high and still fold when you are fairly sure you are being bluffed. But, a lot of value comes from getting comfortable objectively navigating those situations.