1) I sometime look very serious if I am concentrated on my work.
2) I can be frustrated sometimes when working with people who are not coorporative
May lead to question: So how do you deal with it?
Answer: I am pretty good at handling my frustrations. I would take a break, get some fresh air, or work on something else to quickly calm down myself and deal with the issue again with a clear mind.
I would sit down with that person to find out why things don't work out, clarify some possible misunderstandings, discuss solutions, and follow up closely to ensure all things are properly done.
3) I am good at XXX as I have lots of experience working in this area. I may be a bit green in YYY as I haven't had a lot of chances to work on that, but I am sure if given this position, I am a fast learner and I should be able to pick it up quickly.