Really good advice from Juno- ie speak to students in the lab that you are interested in joining- they are the best source of advice/info/help etc and can let you know if the supervisor is a 'hands on' or 'hands off' kind of person. These students should also be able to tell you how much personal input/control you will have over the direction of your PhD. Bearing this in mind, every project is different, even under the supervision of the same person. For example, my project was really well structured and thought out before I even started,whereas other students in the same lab start with a fuzzy kind of vague idea but no real plan of action... it really does depend on a lot of things... And it depends if you will be field work based or lab based, and how much you already know how to do- e,g a lab based project will involve a lot of training/help at the beginning (which will most likely come from a Post doc)...
hope this helps!