NPCs can act on their own initiative, but sometimes a PC has to give them the order. They can hold back for some reason, or wait for confirmation. Put the onus on the PCs to make game-critical calls, to make sure they stay involved.
Admiral Ackbar: You're closing in on the innermost yellow bits!
Wedge: We've penetrated the Peace Moon reactor chamber. Target sighted!
R2-D2: The Lost Orb?
Wedge: Shall we blow it up?
C-3PO: Um... I think we have to. Regardless of what... or who... might go up with it.
[SFX]: Pow! Pow!
Lando: Race you to the exit, Wedge!
Wedge: You're on, good buddy!
Lando: Last one out's a singed space sausage!
Admiral Ackbar: I hope you don't hit a snag.
C-3PO: Luke, I'm sorry...
Luke: Well, you know. Out of the frying pan...