I love insta-buddies. I have dove with them since I was first certified and found that only one or two presented problems like the guy who liked to wrestle sharks. We only did one dive together.
Friendly communication from the start is imperative. Demands about distances, signs to use and what not often overwhelm the unsuspecting insta-buddy and might make them avoid you underwater. Be nice and ASK them how they want to do the dive, then adapt. "How close do you want to stick together?" "When do you want to surface?" "What do you want to look for?" IOW, make the dive about them and see how much they get into it. Too often I see the newbie diver trying to get the other diver conform to their needs and they can get quite frustrated doing so.
Secondly, relax! The worst thing you can do before a dive is to get all anxious. It's not good for you, your buddy or your SAC rate!.
Relax and have fun. Don't make mountains out of mole hills. With hold any commentary (suggestions) unless they ask you for them. I learn from everyone I dive with. Sometimes I learn things to emulate while other times I learn things to avoid. The best thing I learned is to stop telling everyone else how to dive. If they want my opinion, they'll ask. I only speak up if I see a dangerous situation developing.
About those in-water emergencies, what you don't practice can hurt you. Try a gentle CESA, but stop at 15 ft!!! This is especially easy on a line. You might be surprised at how slow you really have to go if you're calm about it. Also, do some air drills, some no mask drills and even swim back to the boat with only one fin on from time to time. Do these if the dive somehow seems un-challenging (I'll infer boring here) or devoid of purpose. No, you don't have to practice on every dive... but don't forget to hone those skills for when you do need them.
BTW, remember my second rule of diving: You can call a dive at any time and for any reason: NO QUESTIONS ASKED! When I get put with an insta-buddy who is more an insta-nightmare, I simply call the dive. No reason for me to get distressed or worse, to be in peril. Often, I'll ask the DM for a new buddy or I might even sit out one or more dives. It's all good. I love being on the ocean in any event, so I haven't lost much.