One thing to add to these great answers, would be to learn to take cues from others when in the workplace or networking. There are different communication styles and some people are task focused rather than relationship focused, nothing wrong with that! Don't be offended if some people speed through the chit chat and go into the project or subject at hand. Always be true to you, if you have a story or a quip to throw in, add it! Just make sure it doesn't feel forced.
I think being friendly helps in many situations. For instance, if you are networking, being too friendly is not a bad thing, since people will welcome the attention you give them in a crowded room. Same thing with sales, if you're not upbeat and positive about what you're selling, then people will not be excited about what you are trying to sell them. I recently had that experience and realized I needed to change my pitch, because my relaxed pitch wasn't exciting people. And I agree with Michael above; in an interview, you want to be friendly and let them know you are someone who they will be happy to work with.
Great question Angelina. I think the golden rule applies here. I try to treat others the way I want to be treated as much as possible. Most people, including interviewers, like warm and friendly, especially in prospective employees. A negative, unfriendly attitude will rarely help out in a job situation. Be upbeat and Energetic. Smile even if it's a phone interview. On the other hand, I suppose one could overdo it with friendliness, but I do not think you need to worry about that if you focus on answering questions honestly and sincerely with a smile.