According to Madara, someone only has to know the proper hand signs in order to release oneself from Edo Tensei. I don't necessarily think this applies to being controlled, though, since that is dictated by a separate tag that is unique to the caster (Orochimaru and Kabuto used different sealing tags). I think this primarily applies to the sealing of the soul to the body. If you noticed, Madara only released himself when the technique was release by Kabuto.
Additionally, the ability to release one's self from Edo Tensei also can be limited by the amount of free will that person has. Kabuto gave Madara nearly complete control over himself. Whereas, Orochimaru was in total control over the Hokage when they fought the Third.