What's the difference between SpeakString and ActionSpeakString?

I believe ActionSpeakString goes in your action queue, so it can be delayed while other actions in the queue happen, like walking over to the person you're going to speak to.

From what I've found, ActionSpeakString speaks down below in the chat box, like a character shouting or whispering something. SpeakString is a little white one-liner conversation above the object's head. The action makes it as if something has SPOKEN in real-time instead of displaying a message. I may be wrong.

BTA got it right.

SpeakString executes when the script runs.
ActionSpeakString is put in the action queue.

Boo-yah! One score for me! Oh, wait. I was wrong. Eeek.

I think what I was talking about is if you leave the TALKVOLUME blank, theeeeen it appears in white on the game screen. Otherwise it's fish for the chat box. I may be wrong again. Oh, well.

Well, the question was what the difference between the two were Xaelifer.

Both have been useful for me. If you script some character action you would normally use ActionSpeakString. As in
ActionMoveToObject(<some position> );
ActionSpeakString("Hey, you there!!");
ActionMoveToObject(<position of PC> );
ActionStartConversation(<PC object,"a_conversation_of_some_sort")

The SpeakString I have mainly used for debug purpose. But there are situations where ActionSpeakString cannot be used so you have to use SpeakString.