If you're in an RP duel with someone, don't just rattle off spell names. Specify their intended effects and give your opponent time to retaliate. Just because you cast a spell doesn't mean that it's going to be successful. Perhaps your opponent wants to block, evade, or deflect. Even the Killing Curse can be defended against if you're creative enough. By the same token, don't just cast Protego for every spell your opponent throws your way. Make it fun and believable.