I am aware that certain individuals are more prone to adopting the traits of others' in order to merge with other people. I'm also not suggesting that we all just passively absorb the traits of others...
But to what extent have you experienced a more significant and extended 'characteristic change' as a result of being around other personalities?
To answer the question myself;
In my own experience, when I was younger, the effects of other peoples personalities on my own personality was more significant. This correlates with well known developmental psychology which holds that there are more sensitive periods in human development. Anyway, I spent a good deal of time as a dancer, surrounded by very outgoing, confident, spontaneous girls. Now, I didn't become more outgoing over night, but I gradually became more open to the notion of being outgoing. I went from being very withdrawn, to being quite comfortable spending long periods of time with people, and talking about myself to others, over a period of 8-9 years.
Another example, is when I am surrounded by people with traits I don't value. I consciously become aware of what I have decided that I don't want to be, and I hold myself to this resolve. I'm also more aware of the characteristics of mine that may be heading in that direction, and make sure to prevent myself from sliding towards this. For example, I observe how my parents' deal with my sisters, decide they are ineffective, and decide not to align my own interpersonal skills with theirs. I actually think being surrounded by people with traits I don't value, has most helped me to shape my own values, and thus my own characteristics.
So in a way, the traits of mine I am conscious of, and seemingly have conscious control over, I have in some way or another, shaped, based on my perception of the personalities of those around me and whether or not I decide to integrate certain characteristics. I'm not really old enough to conclude that change was truly extended, but I seemingly have maintained a lot of the traits I have adopted over the years. That said, I have also consciously slowly rid myself of some traits I have previously adopted too, because of current experiences. So in that way, my own characteristics seem to be quite fluid and forever remoulding based on my own experiences, perceptions and judgments. Additionally, I seem to be someone who changes slowly and gradually, but surely. Sometimes I feel like a new person everyday.