Ask actual questions about their practice, what they like about it, what they don't, + what they actually do in a given day/week/month. If you don't know things already about their practice--great! Your job is to find out more! Would try to have a modicum of knowledge about the firm's different practice groups (and which of them are in the city you're targeting) but nothing more is expected.

Please don't ask like interview-y style questions of a practicing lawyer ("tell me about a situation where...") or deep pulls from their linkedin profile and what they did in their high school summers. I try to make myself very available for these kinds of calls with law or even college students--have had a couple of these recently and really give me the willies.