In the worst case I'll avoid interaction on purpose.
Usually with people whom I don't feel like I have a connection with at that moment, even though I 'know' them.
Or for having a lack of anything relevant/meaningful to say, or 'cause I'm preoccupied.
For instance, when being in the store I don't always feel the need to turn around for every known face there is (unless we've been friends or something).
Also, sometimes I just really can't force a genuine smile or excited "Hiii *kiss*" for everyone. (I'm not best buds with everyone, don't need to do every person a favor, and pretty honest about it.)
And when at social gatherings, I'll do something on my own for a sec; "sit back and relax" (and retreat for a little while).
But as I said, that's at my worst - socially, that is.