A Guest Liaison is a person on the committee who is assigned to a particular Guest of Honor, to be their point of contact throughout the year and to shepherd them through the convention during the event. The Guest Liaison is responsible on the one hand for communicating the expectations of the convention to the guest and seeing that the convention's needs are met, and on the other hand for communicating the expectations of the guest to the convention and ensuring that the guest's needs are met.
Conventions generally seek to highlight the guest at the convention, and will want bios, bibliographies, appreciations, art, fiction, essays, and so forth for publications (and for an artist, typically badges and t-shirts). The convention also wants to make sure that the attendees have a chance to encounter and ideally interact with the guest, which will mean that the guest is nearly always also one or more of program participant, featured speaker, exhibiting artist, banquet attendee, masquerade judge, autograph table sitter, and so on. Many of these roles require work on the part of the person in them, and the Guest Liaison's job includes facilitating these interactions. For instance, the convention may wish to mount a larger exhibit of an Artist Guest's work than the artist can conveniently bring with them, in which case arranging shipping (and possibly matting and framing) may fall to the liaison. If the convention expects its program participants to sanity-check their schedules (and many do without realizing it) the liaison should take a first pass at this. Even little details matter, like what kind of pen the guest prefers to sign their books with.
Different conventions handle the financial details of being a guest differently, and new guests may not be familiar with expectations here. It is important to make sure that the convention and the guest are on the same page here. Do you reimburse food, grant a per diem, assign committee members to pick up the tab at each meal, or something else? Is alcohol covered? What times is the guest expected to be there? Will there be pre-con signing sessions so that the dealers can have autographed books to sell? If the guest wants to make an extended vacation of it, how many additional days will the convention cover? If there are comp rooms that can only be used on Thursday or Sunday night, make sure the guest understands that they can have some extra time in your city without it costing the convention anything.
If you have a pair of Guests who are a couple, it is fine to have a single liaison for those two guests. Not every convention assigns liaisons to their guests, but if you have the people power to do so you will find it very useful and your guests will appreciate it.
A Guest Liaison should be a responsible person, preferably with their own car. If the liaison does not have a cell phone they should be assigned one (and should have contact info for their guests programmed into it). It is nice to recruit someone who is enthusiastic about that particular GoH, but you want to make sure they will treat them with respect and be able to assist them without being too starstruck. Guest Liaisons should always remember that GoHs are people too.
Optimally, Guest Liaisons should not have any responsibilities during a convention other than to liaise their guest.
At the convention, Guest Liaisons are primarily responsible for making sure that their Guest gets everywhere they need to be on time, and also for making sure the Guest is fed and has all their other needs attended to. This may include transporting the Guest to and from the airport and area restaurants, pharmacies, etc. Making sure a guest gets to program items on time may include extracting them from a group of fans or another type of conversation. The liaison should be prepared to have to watch the clock for their GoH.
Different Guests, of course, require different levels of attention from their liaisons, and different types of accomodation from the convention. If you are trying to figure out how needy a guest is going to be, it can be helpful to speak to someone from another convention where that person has been a guest. Generally speaking, however, the needs of an artist GoH are ten to twenty times as time-consuming as those of other guests as there are crating, shipping, agency, and publications concerns that do not apply to other kinds of guests.
Some conventions have someone who arranges all the travel for their guests, but for many conventions that responsibility falls to the Guest Liaison as well. It is most convenient for the convention if a liaison is prepared to front the money for guest travel and other expenses, and then provide receipts to the convention and get reimbursed.
A convention should provide all liaisons with a schedule of what the convention needs the liaison to do. Different departments may provide the liaison with lists of questions to ask their Guests, as to hotel accomodation preferences, green room needs and desires, publications needs such as bios, CVs, and photos, and programming preferences. Liaisons should follow up with their GoHs to get responses to these requests in the timeline that's needed by the requesting departments.