[maket]

I worked at a fast food place back in high school, I often used this approach to a pretty effective degree. The only issue that can arise is when your increasing friendliness causes the person you're dealing with to get irrationally angry and do something rash, such as throw a cup of coffee at you.

It was pretty painful, but the laughter the situation induced was a nice distraction.

[markc]

I'd imagine that increasing friendliness in such a situation could easily be interpreted as mocking or patronizing (no matter how sincere you are) and in that case the hostile reaction isn't to the friendliness it's to the mis-perception of it.

[zem]

in most cases it is patronising; that's the entire point of how you psychologically "win" the encounter by being friendly to someone who is being a jerk to you.

[marvin]

...which is why using social manipulation techniques like these area great for working with strangers or people you will never see again, but mostly a bad idea in more permanent relationships. E.g. with colleagues, repeat customers that you want to see again etc.