Some would argue there is no role, in that the Candidate has things "done" to them, as opposed to actively paritcipating. However, this is, in itself, a role.
The passivity and receptivity required of the Candidate is their role. They are to become a Vessel for the energies and symbols implanted in them, and thus they must be made passive and receptive. Much of this is acheived through various dramatic elements of ritual, such as the quite potent one of shock. The not knowing what it happening or what is about to happen tends to heighten the senses to a degree that all things done are magnified in force. If, for example, the sight is removed from the Candidate (via a Hoodwink), then when it is restored there is a huge impact, including the fact that a coloured prop will now seem suddenly more vibrant and alive than it would have had the sense of sight not been heightened accordingly. Much of what the Candidate is to say is also said on behalf of him or her by the Hegemon, further inducing the state of passivity, where they, being their Lower Self, are no longer in control, having handed this over to the Officer symbolic of their Higher Self.
The main place where the Candidate actually actively participates is the Oath, for the Hegemon does not answer for him or her here. This suggests that the Oath is taken by the Lower Self, not the Higher Self, for, of course, the Higher Self does not need to take an Oath, for it lives by many of the principles expressed in the Oath already. Usually the Oath is given towards the beginning of the ceremony, thus freeing up the rest to induce the state of passivity again in the Candidate. Even the Oath can aid in this, for messing up the lines, especially when hoodwinked and kneeling before the Hierophant, can jar the confidence of the Candidate and eliminate any remaining elements of egotism that may have been brought into the Hall; this too aids towards the receptive state necessary for the initiation to be successful.
As the Initiate advances through the grades they become more active in them, capping this with their role in Adeptus Minor.