If you do not have gambits set, you can choose your attacks and actions. However, if you have gambits on, the characters with gambits set will attack for themselves, and whatever the gambits tell them to do. You can still tell them what to do, but they'll do it once then resume with their gambits.
Good bye everyone, it's been fun!
Special thanks to ED_, superman4u, and Cross Stinger
Credit for the Siggy goes to shadow slayer