Looking like an ordinary tree
 

Remember in The Wonderful Wizard of Oz, when those trees picked up and threw Dorothy and friends whenever they got near but otherwise gave no indication that they were anything but a line of trees? Or, for those who only saw the movie, when Dorothy and the Scarecrow picked apples from those trees, only to find out that the trees talked and were upset? How do I model that in GURPS? Especially with player characters getting suspicious whenever you ask how they act around an otherwise benign object that no GM will bother bringing up to them unless it is killer?

Re: Looking like an ordinary tree
 

Just mention there are some trees, and leave it at that.

If you want mechanics, then the trees could have an Illusion power (Glamor or not) to appear like ordinary trees. Or an Alternate Form in which they are ordinary trees to all appearances and can withstand inspection. You might also manage with the other shapeshifting powers -- Morph (Cosmetic Only) or Elastic Skin.

Throwing people around might be Extra Arms (which I can't resist calling Extra Limbs in this case), or might be Telekinesis.

Re: Looking like an ordinary tree
 

Mention ordinary objects that are just ordinary objects, regularly.

Re: Looking like an ordinary tree
 

Either a high level of Chameleon (with an appropriate limitation) or a very limited form of the Illusion or Mind Control power (i.e., "Only to mimic the appearance of an ordinary tree" or "Only to make people believe you're an ordinary tree.")

If you want to keep it really low cost, give them the Perk "Looks like an ordinary tree when still and asleep."

I'd go with the Perk myself, especially if the creature is naturally tree-like in appearance.

Re: Looking like an ordinary tree
 

And, maybe, mess with paranoid players minds by describing a couple of perfectly ordinary scenes and letting them go into potential combat mode.

After a few false alarms, the players can choose to continue wasting time and potentially borrowing trouble by traveling as if they were on patrol deep in enemy territory, or they can travel normally until they run into the ambush.

In any case, if you've got truly paranoid players or characters, maybe allow the characters to make Vision rolls, or Vision-based skill rolls, to detect the mobile trees before they attack. But, rule that anything other than a critical success fails to detect the trees as being anything out of the ordinary.

Re: Looking like an ordinary tree
 

You declare that they look like trees. Your basic appearance is a feature, you only have to pay points to change what it is.

On the other hand, someone with appropriate skills (commonly naturalist, possibly also exotic skills if the trees have been transformed) should get a roll to notice an oddity.
Either don't bring up the object (make a roll behind the screen to notice), or make a habit of bringing up red herrings.

Re: Looking like an ordinary tree
 

I generally call for a perk (Deceptive Appearance) if you look like something that will often fool people into mistaken assumptions about your abilities.

Which looks like a tree but attacks people should qualify for. It's essentially a free chance to make a Camouflage roll (or even an automatic success) in appropriate circumstances, which certainly ought to be worth a point or two.

Re: Looking like an ordinary tree
 

If you set up the context so that it is likely that PCs would step near the trees (i.e., walking through a forest), then you could just roll a die to see which PC steps near the particular tree, and then run the encounter from there: "Balthazar, as you walk by an otherwise unremarkable oak, you are shocked to see a branch swing down and try to grab you!" I avoid treating the PCs as puppets, but most players I've gamed with are actually quite happy when their characters can be a conduit for further interesting action.

Another option, if you want them to explicitly game out approaching a particular tree, would be to have an event that pulls them into combat time with a hex map. Maybe they hear sounds over a ridge and you ask them to move their figures on a map where you've marked terrain features (including trees). Some are bound to hide behind trees or move past trees as they position themselves. The distraction could be plot-related — leading to a two-front encounter — or could turn out to be something innocuous. I did this once with some rocks that were actually some kind of earth elemental. One of the PCs actually climbed onto a rock (I don't remember why). It was awesome fun when it stood up underneath him.

Re: Looking like an ordinary tree
 

I seem to have miscommunicated here. I have no issue handling these in games. (I roll Per-based Naturalist rolls to see if characters notice anything amiss with that bush before they’re near enough for it to strike.) However, I am looking for what character traits that Old Man Willow has but a crushroom lacks that are specific to looking innocuous.

Re: Looking like an ordinary tree
 

Baseline appearance really is just a 0-point feature if it doesn't generate any reaction modifiers.

Example: One person is a serial killer who looks like an ordinary Joe and doesn't have disadvantages that require him to foam at the mouth and brandish knives. Another person is an ordinary Joe who doesn't kill people and maybe even has Pacifism (Reluctant Killer). You can't tell the apart at a glance. To do that, you need to speak with them a bit or observe them for a while.

It's the same with plant monsters: Two willow trees, one of them murderous and animate, the other just a tree, are indistinguishable until somebody applies suitable observational skills to the puzzle. To ensure that the murderous tree isn't singled out at random for a spot check, effectively giving the adventurers a passive skill check to notice trouble, Honest Face should suffice.