It might be up to the DM but I'd say as a general rule that your cohort will be under your direct control, or at least they will be if they are with your character. Cohorts don't reduce the XP gained by the party by helping in a fight so having them is a good thing - just make sure they don't die.

Followers are good if you have a use for them. Generally you don't want them fighting, but you could have them making items or something while you are adventuring.