Generally I let the PCs run things, especially during combat. Role-playing is left to me, and I also hold a veto on suicidal actions. For convinience, I have pets and famliairs act on the same turn as their owner, but henchmen/followers have their own turn. Non-henchmen/followers are still controlled by me.
If the player tries to abuse this, such as by having the familiar explore an entire adventure area (often resulting in death, dismemberment, or other horrible outcomes), the familiar will become agitated, object (if able to speak), then eventually flat out refuse (berating the PC if able to speak). Hasn't happened yet, mostly because I have an awesome group, but I'm prepared for it.