I start with the dominant trait and then work out what other traits could go with it. I then think about how someone would speak, what kind of language (good or bad!) they'd use, whether they got on well with others or not (and look at the reasons for this). The physical appearance of the character I tend to fill in towards the end as it has never struck me as the most important thing about a character. Well, okay, I can think of one exception to that - King Kong! His physical appearance is pretty important to the story but that one has been written (!) and I still believe that in the vast majority of cases, the physical description is "the icing on the cake". You do need the "solid stuff" - the character traits, what these people are like - first.
I also think about whether the character has a sense of humour or not, as that alone can tell you so much about someone. So what aspects do you consider first when creating your characters? The lovely thing with this is there is no one correct way. I find it fascinating to discover just what characters are made of (my own and others!).