We'd like to think of our personality traits as robust and defining. The characters writers create have stable and clear personality traits--that's what makes them memorable to the reader. If you read a good novel, that certainly seems to be what real people are like.
But it seems that many of our own best traits are really tiny, weak muscles, like the little finger's wonderfully named flexor digiti minimi brevis, easily exhausted and thus useless when overused.
Sometimes a dose of glucose can revive them. Our willpower can get stronger, as can our control over expression of some of our less acceptable personality traits, like racial prejudice, as expressed by various celebrities in stressed situations.
The latter issue, about racist tendencies becoming more pronounced with tiredness or other fontal lobe inhibiting states, has some interesting features. The linked article, "I'm only racist when I'm drunk" (from the excellent Mindhacks) says
The idea is not that all of these people are racist, but that we have absorbed negative cultural associations that tend to push our thinking in the direction of prejudice and we need to make a conscious effort to act even-handedly to counter-balance this tendency.
Is a character who uses the frontal lobe to inhibit some satisficing heuristics that happen to stereotype ethnic and racial groups admirable, or not? Remember, those heuristics just have to work somewhat better than random chance to be reinforced. Stereotypes certainly do make life easier. In previous eras they were used unashamedly and openly. Refusal to subscribe to known prejudices made you, in fact, suspect and worthy of contempt.
So our characters struggle against this side of themselves--though it's tempting to make the admirable characters be the ones who do not have any such internal contradictions. Sober, untired, fully glucosed, the character has the resources to devote to distinguishing individuals and judging them as such. Drunk, tired, hungry, the character uses lazy hacks and hurts and offends others by sampling too infrequently. Do we dare to portray someone that way? Where does the actual "character" live?
Of course, yelling racist epithets--"spewing" is pretty much the de rigueur verb here, which is why I'm avoiding it--is something affirmative, not merely making errors of inappropriate categorization. Such a character affirmatively dislikes individuals who fall into a certain category. Again, expression of that dislike is controlled, until that frontal lobe minimi brevis just gives out from the day's demands.
So, maybe celebrities who know that they might lose control while on camera (which is now pretty much all the time) should keep an emergency vial of sugar water handy. It just might save their career.