The interest in character traits is in the breach.

In most stories the heroes make mistakes, they have fatal flaws, and they must overcome difficulties. Part of the interest in stories comes from the development of the main characters, and their movement from one point to another - which is usually the result of the characters changing and learning from mistakes.In gaming many PCs become quite static, the PCs resist change and the characters start to seem two dimensional. This is frustrating for both the player and the GM.

As a GM it can be important to remember that the PCs are clinging to their concepts for a reason. When players create concepts they usually have in mind a series of events which will challenge and highlight their character traits, in my experience they will cling to these traits until they have been sufficiently highlighted, and then they will move on.

As far as I can see, one of the best ways to highlight character traits is to give the PCs the opportunity to be "forced" to break them. (I use the quotation marks because they don't necessarily want to be forced into a corner as players, but they may like to roleplay their characters out of corners created by the character's personality.)

For example if one of the PCs is always polite, the player is probably thinking about how freaky they would be when they finally snap and get angry. They don't want them to be angry all the time, but they probably would be grateful for an excuse to have that character's self control waver briefly - try giving them something to get really pissed at. Or, perhaps one of the characters is very callous and cold. The player might be interested in seeing what would happen if they were presented with something they cared about (eg. the assassin rescuing the small child).

Remember, the same goes for NPCs. They will seem much more interesting if they occasionally break established character traits. For example, Darth Vader was so cool because he eventually decided that he wasn't evil, and saved Luke. Additionally, endless action movies have been based on the idea of the good, kind normal guy who goes nuts and takes horrible bloody revenge on the people who've pissed him off (usually by killing his family or something). Both of these concepts are interesting because they break the pre established character traits.

Ultimately, some players will reject or fail to see these chances to express their characters (the polite PC might never snap, the assassin might kill the child) but many will jump wholeheartedly on the chance to express that element of their concept. Some players will make these opportunities all on their own (wouldn't we all like more of them!) but most will need these opportunities to be presented, and once they have expressed this element of their character to their satisfaction the character will probably become much more dynamic.