These lines in the character's script file found in Bridge/Characters (ex. Saffi.py) control the base model and textures used by the character:
# Create the character
App.g_kModelManager.LoadModel(CharacterPaths.g_pcBodyNIFPath + "BodyFemM/BodyFemM.nif", "Bip01")
App.g_kModelManager.LoadModel(CharacterPaths.g_pcHeadNIFPath + "HeadSaffi/saffi_head.nif", None)
pSaffi = App.CharacterClass_Create(CharacterPaths.g_pcBodyNIFPath + "BodyFemM/BodyFemM.nif", CharacterPaths.g_pcHeadNIFPath + "HeadSaffi/saffi_head.nif", 1)
pSaffi.ReplaceBodyAndHead(CharacterPaths.g_pcBodyTexPath + "BodyFemS/FedFemRed_body.tga", CharacterPaths.g_pcHeadTexPath + "HeadSaffi/saffi_head.tga")
these control the textures used by the face when the character uses an animated facial movement such as speaking or blinking:
# Load Saffi's general dialogue lines.
LoadSounds()
pSaffi.AddFacialImage("Blink0", CharacterPaths.g_pcHeadTexPath + "HeadSaffi/Saffi_head_blink1.tga")
pSaffi.AddFacialImage("Blink1", CharacterPaths.g_pcHeadTexPath + "HeadSaffi/Saffi_head_blink2.tga")
pSaffi.AddFacialImage("Blink2", CharacterPaths.g_pcHeadTexPath + "HeadSaffi/Saffi_head_eyesclosed.tga")
pSaffi.SetBlinkStages(3)
pSaffi.AddFacialImage("SpeakA", CharacterPaths.g_pcHeadTexPath + "HeadSaffi/saffi_head_a.tga")
pSaffi.AddFacialImage("SpeakE", CharacterPaths.g_pcHeadTexPath + "HeadSaffi/saffi_head_e.tga")
pSaffi.AddFacialImage("SpeakU", CharacterPaths.g_pcHeadTexPath + "HeadSaffi/saffi_head_u.tga")
pSaffi.SetAnimatedSpeaking(1)
just change the .nif and .tga files to those of whatever character you want.? keep in mind that the extras do not have any facial animations so you will have to create those textures.