What does it really mean to know your characters?
It's not enough just to think up a character or even do a profile on that character. You need to be able to walk in your character's shoes, think what they think, react how they would. Simply knowing how you would react isn't enough and would defeat the purpose of creating the character in the first place. The character is a separate person from you and will react and think based on the history and background that you have created for them.
For example, if you have a (character) man who, as a child, moved from foster home to foster home, then his reaction to relationships might be tentative and untrusting. He might become strangely hostile when he finds a woman who wants to get more serious.
On the flip side, let's say you have a man who grew up with the story book childhood, great parents, nice home, well taken care of. How he reacts to relationships would be substantially different and he might even be a bit naive.
This doesn't just go for relationships. This means reacting to any situation. How does he/she react to having a gun pointed at their head? If your character grew up in a gang neighborhood, the sight of the gun might not have as much of a strong surprised reaction as someone who had grown up away from such atrocities.
If you don't write your character's reactions accordingly, your audience will pick up on it.