If you are angry, then we have to admit that as an emotion.  The trick is to have an expression that is appeasing to our attacker, and claim/admit how we feel inside.  I haven't mastered what I am suggesting. but I truely believe that is the trick to survival in the workplace.
Actually, I am quite quick with a sharp tounged comeback and that is what actually gets me written up.  (These just fall out of my mouth before I think about the consequences/or sometimes I think, weigh the consequence and decide they need to hear how inept they are.) I refuse to meet with the person who has offended me at the supervisor's request.  I tell the Supervisor, that it is pointless for me to try to make amends with someone who is obviously the type of person that would go to the level they did to be offensive, and I am not going to pretend to forgive them when I have not.  The offender has shown their true colors, I will no longer even look at them.  This of course makes it quite difficult for the supervisors to run a cohesive program.  I try to not have any facial expressions most of the time and have been called in for being unfriendly and not making eye contact or smiling.  I hate to be told how to hold my face for others needs.  Obviously I am still quite angry at the NT work world, and not sure I am wanting to practice facial expressions in a mirror to appease them.
It's interesting reading some of the replies on here. It is really bizarre timing, however I've started watching the TV show "Lie to Me" - the one with Tim Roth that is all about facial expressions and microexpressions. One of the things that I've been reading up on the net all weekend is about recognising facial expressions and the emotions that are associated with them. I think it's actually giving me some insight into the issues that I'm having at work. The feeling that I think I feel when someone confronts me, or asks me to do something that is either unexpected, new or out of my routine, is most likely worry or possibly fear (i.e., anxiety). However, I believe that what I look like when I'm anxious appears to other people as angry.

I need to focus on the way I appear to others, however sometimes I think I might actually be angry. Its the usual kind of thing, with co-workers who I believe to be less skilled and efficient than me being promoted just because they have better social skills when interacting with upper management.