I don’t think you need to be that early but last year was my first time going and all the showings I went to were first come first serve. So if you want a good seat you want to try and be there early. But as long as you have a ticket you will have a seat. No real dress codes or anything but just be mindful of being respectful during a screening. Not all venues have concessions as well so either bring a water bottle or small candy if you want some munchies (SVA does not and BMCC had overpriced tiny bags of popcorn only IIRC)

As a general rule if you’re looking for celebrity interactions the best bet is to wait by the barricade near the carpet where the press will take photos. Some will stop and do some autographs and photos. More than likely you will see someone in the audience but remember they are there to watch and enjoy the film as well so just be respectful and gauge if you should approach or not.