Smiling too much is not really necessary. I don't do it much myself, unless I'm at a point where there's a lot of comfort already built.
Not smiling but being very attentive and interactive give the impression you "know what you're doing". In other words, it gives the impression you don't have a problem talking to women.
James Bond (as mentioned earlier) is a great example.
Don't be serious either, cause it creeps people out. Also, when you smile, don't show too much teeth. If you overdo it, it can give the impression you're nervous.
Hope it helps.

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