No, circle casting is not always necessary to cast a spell. But I have noticed over the years that the spells I cast within a circle are more successful in a shorter length of time. One explanation for this is that it's easier to raise and send energy within the confines of the circle-- sacred space.
Not every spell you cast will "work", and not every method of spell casting will be your forte. Experiment, practice, read and gather more knowledge. You will eventually find out what works for you.
You can find more information on spells and spell casting at my website, The Witch's Corner, on the spells & potions page...
http://ladyamythyst.webs.com/spellspotions.htm
Good luck and happy spell crafting.
