So, you are ready to choose an agent to sell your home or to help you in your purchase of a new one. 
How are you going to pick an agent? Your boss's sister's cousin's wife is a realtor. She should be good, no? Your nephew just got his license. Another good choice, yes? You know the big-name agent in the area. He has sent you mailings, knocked on your door and sent you Christmas goodies for years. The natural choice of course?
Anyone of these people may be the perfect choice, but beware! This person is not only your representative, they are your ambassador!
If you were a nation you would want to make sure that the ambassador you hire to represent you abroad is knowledgeable, sensitive and good with all kinds of people. Someone who can negotiate with tact. Someone who does not delegate work that they have been hired to do. Someone who knows the laws of the country. Someone who can see each project through to a successful end without inflaming the other side and without inciting rebellion.
This is the person you should choose for your realtor.
The persons front and center in real estate transactions in California are the agents. Generally the buyer and seller never meet each other. All the negotiations prior to and during escrow are handled by these agents. The right agent can make things happen. The wrong one can kill a deal. The thing an agent dreads most is dealing with a difficult or incompetent agent at the other end. It makes an already stressful process even more so.
The California purchase agreement is a not a complicated eight-page document in its basic form, but it does have to be filled out correctly to avoid problems later. Not checking a box or not filling in a blank space can sometimes cause a huge problem later. Not following up also can lose you the deal. Your newbie agent can be fine as long as they assure you that they have an experienced person working closely with them. This goes for the entire escrow process where all the t's need to be crossed and i's dotted. 
That is the practical side. The personality of the agent is also important. Are they pleasant? Are they a people person? Do they interact well with others? Especially when you are the buyer, if you are in competition with others on a property, don't you think the seller's agent is going to lean towards someone they are going to be able to work with for the next 30+ days? Things go so much smoother when the two agents work well with and respect each other.
I will tell you I was in a deal once, as the seller's agent, where the buyer's agent wined and dined me. That was Dr. Jekyll. Once we started escrow Mr. Hyde came out and it was an entirely miserable process. The agent reflected badly on the buyer. I wondered if he knew about this other side was also fooled.
This business is a people business, and a complicated business, not to be taken lightly. Agents get paid well for their work. Make sure their work is good. Choose wisely!
I don't usually lose listings on anything other than price (I'm frequently lower than the other guy), but I heard a new one last week. A very good client referred me to a seller who had tried to sell his $1,000,000+ home last year. I'd be a good listing agent for this home. I had shown the home last year and I have other listings nearby. Our luxury marketing program is extremely strong. My sales numbers are very good right now. I live five minutes from the house. When we met Mr. Seller said, "my wife works out with the other agent five times a week, and despite the fact that the other agent is part time and lives half an hour away, that's who she wants to go with". I was speechless, which is pretty unusual for me. I'm sorry, I just can't compete with treadmill time!