It really helps. If you know your characters well (and I mean REALLY well), writing from their point of view becomes exceedingly easy – If you are writing a story and you hit a scene that leaves you, as a writer, stumbling over how the character might respond, it means you don’t know that character well enough, and THAT means you’re not allowing your readers to know that character well enough to care.
Writing from deep within their point of view is what makes a character come alive, particularly when you are expressing their innermost thoughts, beliefs and acceptances in a familiar and easy style that is more a backdrop than a statement. Who they are shows in their manner, their reactions, their dialogue and stance, and it is all there for the writer if they truly know their characters.
It cannot be hit or miss. You have to know. If you want consistency, you have to make it work, meaning that it has to be there in the first place for you to draw on, or all you’re doing is guessing. Guessing might sound easier to do, but you are going to end up with a bland character (or a bunch of them) or, worse, one that contradicts him/herself.
Not so long ago, I read a book that had two main characters basically ripping each other’s clothes off all the time. The sex was very well described, although the relationship was shallow and the plot was very lightweight with the supposed storyline taking care of itself with no interaction from the characters (who were too busy screwing).