Skeleton

A skeleton is an invisible part of a 3D model, used for the purpose of animation. A skeleton consists of several bones, connected at joints. A model with a different skeleton from another model cannot use that other model's animations; this is referred to as "having an incompatible skeleton". Sometimes skeleton incompatibility stems from "bone order", or the order in which bones are defined in the model file.

Lego Star Wars I, II, and TCS
As a general rule of thumb, most original trilogy characters are compatible, but prequel trilogy characters are hit-or-miss. A full list of which characters have which skeleton can be found here.

List of non-vehicular skeletons:
 * Anakin (Boy)
 * Anakin Skywalker
 * Astromech droid
 * Boba Fett (Boy)
 * Clone
 * Disguised Clone
 * GenericJedi
 * Geonosian
 * Gungan
 * Jango Fett
 * Kaminoan
 * LSWII
 * LSWII shorty
 * LSWII w/ Jedi cape
 * Mace Windu
 * Padmé
 * Padmé (Geonosis)
 * PadméPilot
 * Protocol droid
 * Queen Amidala
 * Qui-Gon Jinn
 * Rep Teer
 * RoyalGuard
 * Watto
 * Wookiee
 * Yoda

Lego Indiana Jones 1
Most non-vehicular characters in LIJ1 use the LIJ1 skeleton, except short characters, whom use the LIJ1 shorty skeleton, and Star Wars characters other than Han Solo, whom use the same skeleton they had in TCS.

Lego Batman 1
A spreadsheet with skeleton information for LB1 characters can be found here.

The following characters use the LIJ1 skeleton: The following characters use the LIJ1 w/ Killer Moth wings skeleton: The following characters use the LIJ1 w/ Man-Bat wings skeleton: The following characters use the LSWII w/ Batgirl hair and Batman cape skeleton: The following characters use the LSWII w/ Batman cape skeleton: The following characters use the LSWII w/ Robin cape skeleton: The following characters use the LSWII w/ Poison Ivy hair skeleton: The following characters use the LSWII w/ jestter hat skeleton: The following characters use the Penguin skeleton: The following characters use the Jack-in-a-box skeleton: All other non-vehicular characters use the LSWII skeleton.
 * Alfred
 * Nightwing
 * Policeman
 * Commissioner Gordon
 * Civvy
 * Gotham Citizens
 * Mad Hatter
 * Killer Moth
 * Man-Bat
 * Batgirl
 * Batman
 * Robin
 * Weirdo
 * Poison Ivy
 * Rock Chick
 * Harley Quinn
 * Penguin Bomb
 * Penguin Goon
 * Jack-in-a-box