Base Actor
generic_actor,
They're used quite a bit in Half-Life 2, take Lamarr for example, she's just a normal headcrab right? Nope! there's no npc_lamarr and they didnt use a npc_headcrab - that's for sure! if you look at lamarr she has no teeth, but a normal headcrab? they do! they've used a generic_actor and selected a model and BAM, there's lamarr! when browsing for the model you want just type in lamarr and you'll get lamarr.mdl!
all you need are some scripted_sequences, possibly a logic_choreographedscene here and there and voila! you've got an actor :P of course, a side note on logic_choreographedscene's, you'll need to browse through the sourcesounds.gcf with GCFScape which you can find on a search engine. in order to find the right sounds, go through 'HL2 > Sound > VO' - you can find all your voice acting there, you may find the C17 ones in 'trainyard'.
Enjoy,
-Rasirez