As a stage manager, I would try very hard to talk my director out of doing a cast-inclusive talkback at intermission or preshow. There are a few circumstances under which I'd make an exception (if you had a large school group in and they had to get on a bus to zoom to the airport immediately after the show, for example), but it's just such a bad idea all-around. (Doing it preshow means that you need to vacate the theatre earlier, you need FoH and technical staff for an extra 30-45 minutes, actors might miss out on prep time or preshow rituals which matter to them, and anyone who arrives on time will miss the talkback. Doing it at intermission completely undermines the point of intermission [get out of your seat, go pee, get a drink, answer your cellphone, have a smoke, and otherwise do those things which the mean nasty ushers won't let you do during the performance], robs the cast and crew of their breaks, makes life difficult if the show has substantial inter-act costume or set changes and seems like the weakest time to do it: preshow would be better for dramaturgical information of the "keep an eye out for this" variety, while post-show works best in terms of answering questions and dealing with audience reactions and perspectives.)
If it must be done at intermission, I would try very hard to convince the director to bring in a company outsider (the company librarian, the artistic director him- or herself, the designers [if they're on staff], a Professor or Theatre or Literature [or History, if appropriate], etc.) rather than forcing the cast to do it.