/jp/ - Otaku Culture

DJT please help me out.
When you check words on jisho, for example 生きる it usually shows "other forms" which means the same word written with a different kanji right? like 活きる　in this case.

Now the question is, are all forms equally viable in everyday use, or are they only used in specific situations?

Another example would be 守る　護る　why would you use the latter one? cause it looks more complex or is it the old way to write the word and thus is used to carry over some different feel when reading something?

Please help me to solve this mystery.