Details are very important in character creation, they could make or break your character! There won't be tips on little details, since there can be so many, but there will be one thing: When planning details, be careful, they will change your character a lot!
For example, a reader can tell that a character is impatient if she taps her feet from time to time.
It is especially important for the characters to have little details as this makes the reader able to relate to them more. Let's take the antagonist for instance, say he's a mean, nasty, bloodthirsty man, that's alright but what makes him special? What makes him stand out in the sea of all other such people?
Maybe it's his love for roses or unrequited love from someone he has been chasing after or ability to create criminally beautiful art.
If we have a protagonist whom you've thoroughly ascertained is no Mary Sue, how do we give them a personal feel, it must be something that readers can relate to; perhaps he's reckless or likes to hang his coat on the third peg or only drinks a certain brand of tea.
As said above, little details can make or break a character however, do be sure not to overload your characters. Think of ordinary people then apply that to your works.