You will be told who your Overseers are at the briefing session and the information will also be posted on the project web page.2Overseers are intended to provide impartial advice and are allocated so that nobody is simultaneously both an Overseer and Supervisor for any candidate. Overseers are available for (reasonable amounts of) discussion from the time of the briefing session up until the day on which proposals are submitted. During that critical planning period you have support both from two members of the Laboratory staff and from your College-organised Supervisor and Director of Studies.
When project proposals have been formulated, it is the Overseers who check them and recommend their acceptance to the Head of the Laboratory. Before submission, candidates must have talked to their Overseers about their ideas for projects and obtained informal acceptance of their plans based on near-final drafts of their proposals (see Section 6.8 for a detailed timetable). This ensures that the checking and formal approval processes will not cause trouble. It makes sense to give your Overseers the best possible chances of checking your plans early, and to take account of any issues that they raise. You will probably have most of your discussions with just one Overseer but you should send copies of draft proposals to both, since both will have to approve your final plan.
Your Overseers will need to be convinced that you understand the proposal, that it is a sound basis for a project without being too ambitious, that any special resources that will be required while carrying it out will be available, and that the proposal contains a suitably detailed work-plan with a timetable and list of milestones. When they accept a project they are agreeing that the description of it in the proposal is adequately detailed and that a reasonable candidate could complete a satisfactory piece of work given that specification. However, your Overseers will not have detailed knowledge of your particular strengths, weaknesses and background, so they are not in a position to certify that a project will be a great success for you in particular; of course they will be prepared to talk about such issues if you ask them.
The most efficient way to communicate with your Overseers is by e-mail. In this way you can send them drafts of your proposal and they can return comments much more quickly than by chasing each other around the Laboratory.
Overseers are not expected to invent projects, nor do they (in general) provide advice once project proposals have been submitted. However, up until the proposal submission date they can provide useful advice and help.