Experience with Doppelgangers? Help please.

[madwolf10]

Tried searching, went through 6 of 20 pages of results with nothing that helps me...

So, whether as a Player or DM (in 5e, though not sure that matters here), what are the ways that PCs could identify a doppelganger acting as an NPC that one or more of the PCs have some basic knowledge of?

Assuming the Dopp did his Read Thoughts job adequately, he would know things like left-or-right handed, food likes/dislikes, typical daily activities/routines, etc.
Anything more, or more clever, someone has used or can think of?

Also, is this "taking their form" of the Dopp "perfect?" Like, down to every birthmark, scar, recent bruise, recent limp, etc etc ?? (cuz if not 100%, this could help me)

I realize the Dopp can't fight, cast, do special skills/abilities of the normal NPC so there is that, but this may be more of a social setting so no idea if those would come to light...

Obviously I want to dangle a few "things seeming off" to perhaps motivate the party to start questioning what they are seeing.

Thanks in advance!

[Quertus]

The NPC always tells great jokes; the doppleganger has no ranks in perform: jokes.

The NPC has great (or terrible or just "seen by the party") handwriting - the doppleganger writes differently.

The NPC feels certain emotions for the party members - the doppleganger has to constantly fake it.

The NPC *feels* a certain way about his clothes, his possessions, etc - that the party may not be consciously aware of.

The NPC is skilled with a yo-yo and constantly playing with it - the doppleganger has no skill with said device.

The NPC knows things that the party is asking about - the doppleganger has to fake it and come up with random answers.

The NPC has a smoking habit - the doppleganger hacks and turns green.

All in all, (3e) sense motive checks would be in order, difficulty depending on how obviously poorly the doppleganger fits in his new role, and how attentive the party is.

[madwolf10]

The NPC always tells great jokes; the doppleganger has no ranks in perform: jokes.

The NPC has great (or terrible or just "seen by the party") handwriting - the doppleganger writes differently.

The NPC feels certain emotions for the party members - the doppleganger has to constantly fake it.

The NPC *feels* a certain way about his clothes, his possessions, etc - that the party may not be consciously aware of.

The NPC is skilled with a yo-yo and constantly playing with it - the doppleganger has no skill with said device.

The NPC knows things that the party is asking about - the doppleganger has to fake it and come up with random answers.

The NPC has a smoking habit - the doppleganger hacks and turns green.

All in all, (3e) sense motive checks would be in order, difficulty depending on how obviously poorly the doppleganger fits in his new role, and how attentive the party is.

Thanks, Quertus, all good thoughts. Though tbh, some of these, if I played the Dopp wise enough, could easily be researched, known and adopted to look correct.. if he did his job right But this is the right line of brainstorming!

[Kelb_Panthera]

I don't know exactly how they're done in 5e but in 3e there are fairly frequent opportunities to catch a doppleganger.

First, disguising itself as someone specific entails a disguise check albeit with a +10 bonus for shapechanging magic. Those who are familiar to the impersonated character get a bonus on their spot check to penetrate the disguise ranging from +2 for a minor aquaintance or sometimes business associate to +10 for an intimate partner.

Then the observers get to check sense motive against bluff every time the doppleganger tells a lie or acts out of character for the impersonated as well as anytime the player feels particularly suspicious and calls for an active sense motive check for a hunch. This doesn't necessarily reveal that there is an imposter but can lead to pointed investigation and magical scanning which may reveal the truth of the matter.

All of this goes right out the window for the greater doppelganger but I don't think they've been adapted to 5e yet.

In summary, the disguise is never absolutely perfect and doing anything out of character or speaking of events it doesn't know about first hand can give a doppelganger away.

[Demidos]

This may not be strictly RAW, but add some sort of distiguishing characteristic that they always carry with them, regardless of their form. You see this in folk tales, where the Devil always has cloven feet or slitted pupils, but it could also be something less noticeable like a limp, differently colored eyes, or a certain birthmark. You could reveal this with spot checks, and then it would be up to them to realize that the person has been acting ever so oddly.