I would look at either Corrade or MetaBOLT.
MetaBOLT is no longer supported by it's creator, so recent changes in SL have broken a few features (like scripting costume changes) but it has two features that make it easy for operating NPCs. It still interfaces nicely with LSL (with the LSL-API) for the commands/functions that still work (that is if you can still find documentation on the commands available in LSL-API-- I suggest using the Wayback Machine for that information.). However, if you're not an LSL scripter looking for a machinima client, you can use the METAnomy feature to easily set up your scenes. MetaBOLT was originally designed as a text viewer, but as more users asked for NPC/group bot functions, it became very flexible bot client with training wheels: you still have text-based feedback, and a few interesting features available, that easily allow you to monitor your bot's environment. MetaBOLT is a lot lighter (CPU/RAM) to use than any of the graphics viewers.
However, if you're looking for light, and need more than just a few NPCs/actors....
Corrade is incredibly lighter and far more flexible than MetaBOLT or Radegast. It's list of commands are overwhelming (NPCs can be scripted to "MARRY" and "DIVORCE", so if you're looking to generate some controversial buzz over your machinima project, Corrade gives you some options). If you're simply looking to animate NPCs you'll only need to be familiar with commands like "SIT", "STAND", "TYPING" (which is useful for handheld props that replace the default typing animations), "WEAR" and "UNWEAR" (for costume changes), TOUCH (for avatar/object interactions), ANIMATION, AUTOPILOT, TURN, and TURNTO (coupled with a good AO). Even though the MP has a few items released by the Corrade team to help you get started, you'll need to really brush up on your LSL skills to get your actors to perform.
If you're not a strong scripter, and need NPCs for a short term project, I suggest using MetaBOLT. https://wiki.casperdns.com/index.php/METAbolt#Introduction https://bitbucket.org/METAbolt/metabolt.bitbucket.org/overview and you'll want to download the METAnomy plug-in (which is fairly intuitive and easier to use for simple projects than LSL-API) If you need the LSL-API documentation, use "metabolt.net/METAwiki/lslcommands.ashx" in your search at the Wayback Machine. A quick note: all costume changes will have to be done in a graphics based viewer before using your NPCs in MetaBOLT. If you're planning on creating regular features, take a look at Corrade http://grimore.org/secondlife/scripted_agents/corrade
I'd suggest MetaBOLT over Radegast unless you're familiar with RLV programming.