This isn't a question about 'pick-up', but rather, just a social question. I figured that people here have good social skills and that maybe someone could help.

Anyways, my problem is that I always find myself wanting to say something, and ending up keeping my mouth shut because I fear the response. Could be anything from sharply disagreeing with my boss, to responding sarcastically at someone's idiotic insight or asking a guest in my house to move to a different seat, because he's occupied my spot. So how do I become more blunt, or if you will, assertive?

Take the filter off that connects your brain to your mouth.

Just start doing it. Start slowly, use clever words to phrase things. You'll get better as time goes by. Remember, you are not so different! You would be suprised how far you can push things. I've been working on the same myself. Getting better every day!

I used to be the same way then I finally said fck it I am gonna say exactly what I think. Example: last night I was in line to get into a club this homeless guy comes up asks for some money I gave him a dollar. He replies with that's all? You're going in there give me 1/2 your drinking money fkn cheapskate. I had a quarter in my pocket I pegged it off his forehead and was like go fck yourself you worthless piece of sh-t. I know that was harsh, but I said what I thought and didn't take his shit and he left me alone without another word. Like the previous poster said its just a matter of taking your filter off.

Change the way you think. Don't be like "What will I say now?" just think "Fuck it I'm gonna go do this."
Try doing random things to boost your confidence- go to the mall and high five random people. Walk up to strangers and ask them questions. Most importantly- set goals and go higher every day.
Making bets with friends also helps.

I agree. I ve been doing this a lot lately myself, and if nothing else it improves your social skills. If I am at the mall I'll find a cute girl and just talk to her or even if iam at a cross walk I'll strike up a convo with whoever is beside me. Since I've started doing that I've become a lot more confident and I don't have any problems with AA like I used to.