well if you start to feel uneasy and want to say a protection chant, shield, or something similar you can. like you said though, this isn't like in movies, so you won't encounter someone with a staff who blasts a fireball at you and you can defend with lightning or something. if you need to cast a spell, you need to charge energy and focus, with experience it'll become easier, but when you start out, you'll probably have to sit and meditate before trying to say a protection chant or something.
yes and no to more experience equals less casting. everyone's life is different. at first i guess many people find casting spells cool so they cast for every little thing, but i think if people cast less with experience is because they're wiser, and know calling upon deities to help find a lost earring is a waste of time and energy, plus it could make the being you called upon angry with you. i've always followed a simple rule with casting, i wait a week before i cast a spell, unless it's an emergency like a protection spell or a friend asked for me to cast for them.