One of the major obstacles for a writer is to write characters - in plural. Many writers and authors write one character, over and over. S/he's always witty, and handsome, and sharp-minded and willing to meet conflict. Recognise the description?
One way to remember that all characters are characters is to imagine them in different "compartments". These will differ depending on which traits you are looking to give them. For conflict, I have three compartments: See no evil, hear no evil, speak no evil.
See no evil:
Your character might know something is wrong but s/he rather looks the other way and pretends the problem doesn't exist. This usually goes on until something dire happens. This type of character needs support to drive the plot forwards, for example a side-kick desperate to open his/her eyes. They can usually provide very humourous situations, and walk right into scenes most characters would avoid - a cheating partner, face a murderer without back-up, trusting someone they shouldn't, etc.
Hear no evil:
Your character refuses to believe anyone's word that something is amiss but insist on innocent until proven guilty. S/he also can't help but go investigate to find out the real truth. This will get them into problems when they meddle in things they shouldn't - like spying on smugglers, stalking partners, and go visit that growling volcanoe where an angry God is residing set on destroying the world. This type of characters provides very frustrating conversations for side-kicks who can never get them to trust their word. It sets up for double work, and your character will in person go look at everything interesting for plot, which makes it a good main character for a single-POV story.
Speak no evil:
These ones are tricky, because they simply will not say what's bothering them, or what they've seen, to anyone. This demands a lot from the writer to reveal things in body language and how the characters say the most innocent comments. If this isn't the active POV or not an internal narration it proves even more of a challenge. Which is, of course, why I love them. These kind of characters might give hints, and often dodge out of arguments. They will prefer to deal with the business themselves, so they don't need to talk about it with someone else, and probably goes off on their own investigation instead of turning to the police. In a romance story, their unwillingness to say what's wrong will push a relationship to the limit.
These "compartments" can all be mixed up, although I'm not sure I'd like to attempt a character having several of these personality traits! As always with examples, these are extreme generalizations. A character might just as well have one of these traits but consciously try to overcome it. Or just have hints of it in some situations. Either way, I hope this way of thinking will help you to stay on track while writing.
Do you have any 'mind-games' to stay in tune with your characters' personality? What kind of compartments do you put yours in?