What Dozy and what Stuck said. From my experiences, though, lot of it depends on the kind of relationship a brother has with the CO. The key way those relationships are built is when the CO visits the congregation. Some brothers a CO may really hit it off well with, not to mention the previous CO will leave notes for the next CO to give him an idea of the congregation makeup, particulary the elder body. The time spent in field service is taken into consideration too. However, never underestimate the power of brown-nosing cuz I know a couple cats that have done assembly parts with less than national average numbers. However their families have one or two theocratic heavyweights in the circuit, or they've brown-nosed the CO during his visit.
Another thing I've noticed about getting selected to do talks at assemblies, is to be part of the stage crew. Back behind the curtain, you interact with the circuit heavyweights, including the CO. You're there on the stage during the "walk through" in the mornnig and afternoon with the CO/DO, and they're making small talk and joking around to keep it lively. You'll meet the special Bethel speaker who flew in the night before. You'll meet the whomever is handling the finances. You'll meet the other speakers from other congregations. Often the head of the stage crew is a brother who has impeccable theocratic credentials whether that be he's a long time JW, pioneer, Ministerial School graduate, etc.. All this time that you're dealing with these guys, you're building a repoire with them, and if you impress them enough, they'll remember you.
One last thing, is every now and then they'll have the COBOE call a quick elder meeting after the meeting and ask all of the body if they felt there was anybody that could be considered to deliver a talk at the assembly. They do that to avoid the appearance of it being the same brothers giving talks over and over again at the assemblies. To be honest, I still see the same brothers doing talks at Circuit Assemblies. It never changes with the exception of what talk they deliver, or who might be the chairman for that session. It's always the same families or individuals doing interviews, demos, and soliloquies.
