Knowing what to do is important. Demeanor is key; too eager, and they will smell the desperation. Too uninterested, and they will be turned off. How do you act during the phone screen, the in-person interview, and the second (or more) interview?
The key to the phone screen is agreement. The interviewer cannot see your body language, so you have to have a lot of smile in your voice and not use tricky or abstract concepts. Be plain spoken and agreeable. This will help you build rapport with your interviewer. For the second one (your first in-person interview), you show a willingness to do the job, but you do not come off as overeager. You are professional, calm, cool, self-possessed. Everyone is always asking you to do his or her operations or development work (as the case may be), or you are so in demand as a (insert your skill set here).
All this is routine and nothing out of the ordinary. Such requests of your time happen frequently. Therefore, you are receiving their request pleasantly, but you are not excited about it. This happens all the time.
Mimic the interviewer's body language in order to convey agreement without having to move off your low-key stance. You want to smile and be agreeable, but not too agreeable. Serious, but smiling. The more professional you come off as, the better your pay will eventually be.
You are not going for the home run here - only the base hit each time. Only one base too - not two.
The key is the pacing. The company has to pull you in towards them with each step. If the recruiter pushes it too fast, the client gets cold feet and then the deal unravels. They have to take the lead.
You will have no longevity in a place where you had to push yourself in to fit. When they pull you in, they are heavily invested in your being there. Do not "win" the battle only to lose the war.
At the end of the interview, you want to ask for the job. Sometimes they do not register it in their heads until you say it aloud and plainly.
The follow up interviews are similar to the first. Do not make the mistake of relaxing because they now have greater familiarity with you and you with them. Use that they are more at ease with you, but remember that they can still hurt you by rejecting you. So, do not think that you "have" anything going on until you receive a written offer. Do not be emotionally invested in any one company, and keep interviewing until you have the offer you want.