Do I believe all of my clients are innocent?
Obviously, crimes take place, in far greater numbers than people who are arrested for those crimes. Someone does those crimes, and frequently it is easy to solve those crimes. Some incidents are obviously crimes, and obviously took place. If a person turns up dead by gunshot wound to the head, then someone committed a crime by shooting him. If you go to your car and it is gone, and there is broken glass where it used to be, someone committed a crime by stealing it. If you have property missing from your home and a broken window where someone entered, then someone burglarized your home. The same is easily said of someone who shoplifts, possesses or sells drugs, etc.... (this is not to judge the morality or immorality of possessing drugs, something I think should not be handled in the criminal justice system).
These are easy situations. If I shoot someone in cold blood, I've likely committed a murder, but not necessarily. Perhpas I shot in self-defense, perhaps I shot the person after catching them in bed with my wife, and perhaps I shot him incorrectly thinking that he was going to harm me or my family. Maybe I shot him, but it wasn't premeditated. In other words, if I shoot someone in the head and get charged with premeditated murder, I may not be guilty of that for a number of reasons: perhaps I get a complete pass since I acted in reasonable self-defense, perhaps I am guilty of manslaughter because what I thought was self-defense turns out to have been incorrect (we see this all the time when the police shoot someone with a "gun" in their hands that turns out to be a wallet, only they usually don't get charged). The crime may be manslaughter because I acted in heat of passion, or perhaps I shot them, but I didn't premeditate, so it's only a 2nd degree murder instead of first degree murder. These are all things that, as a lawyer, I take into consideration and try to work for when I represent someone charged with a crime. As you can see, there is often a large gap between guilty and innocent in certain offenses.
Just because a client insists that he acted in self-defense, even if there is plenty of evidence to suggest that, it doesn't mean that the prosecution doesn't file the case, or dismisses it. Most prosecutors get blinders on when it comes to self defense (unless the person claiming self-defense is a police officer). They come up with a million places where the defendant could've done something to avoid the confrontation, and focus on each of those things, forgetting the larger scene where the person killed clearly instigated and caused the incident leading to their death. Furthermore, unpopular defendants rarely get the benefit of self-defense in the police or prosecutor's mind. I have had cases with gang member defendants where the prosecution completely ignores and doesn't care about the fact the person who died started the whole confrontation - they view it as two bad people for the price of one, one in jail, and one dead, a twofor.
These are relatively simple examples. They deal mainly with the principles of an act (ie - the shooter, the robber, the thief, the person who actually has the drugs in their hands). A whole area of law exists for "aiders and abettors." These people did not actually do the act, and the only question is whether they aided in the commission of the crime in some manner. Aiding and abetting does not necessarily mean driving the car for a drive-by shooting, it could also mean being the passenger in a car from which a drive by shooting takes place. In my experience, in gang cases, prosecutors will usually file on just about every person in a car that does a drive by, even if there is no evidence that the person knew a drive-by was going to take place, or that they encouraged it in any manner.
One can aid in an offense in many other ways, often innocuous. I have had numerous drug cases where someone is charged with aiding a drug sale by acting as a "lookout." In other words, the police say that they did a buy from someone, and another person stood nearby looking around to make sure that no police were around. This type of case usually hinges on the testimony of a single police officer who makes observations while doing a buy. The biases of these officers are obvious, aside from the fact that as police they are working with people they generally don't like, but they walk into a situation with the preconceived notion that most of the people "hanging around" are doing so for no good reason. They don't live in these types of areas because they don't like the characters that hang around there, and view all of the people in the vicinity in the same light. The other testimony in a case like that would be the opinion of the officer as an "expert."
As I've mentioned earlier, there is no greater scam in the criminal justice system than so-called expert testimony by police officers on things like gangs and drugs. Their "expertise" comes from going to classes and talking to people they arrest. In other words, they are no greater experts than people who watch Homicide or The Wire regularly. However, officers will give opinions cloaked in a mantle of legitemacy of "expertise" to juries that will state whatever conclusion they wish to reach in that particular case. There is no need for consistency, they don't have resumes, writings that can be compared with, regulations or guidelines for their testimony, in other words, they can say whatever they want every time they take the stand. Sometimes you can catch them, but not always. I happened to nail one "expert" in my last trial, but that was pure serendipity.
As you can see, there is a large area to explore in cases, and these are cases in which it is relatively undisputed that a crime took place. Now think about the other large amount of cases where someone accuses someone else of hitting them, or threatening them, where there is no physical proof that a crime actually took place, it is just based on someone's "say so." We have to take their word for it, and this depends on their credibility, bias, and any other factor that could lead one to make up an allegation against someone else (think - Kobe Bryant).
This leaves me with plenty to do that doesn't suggest that someone is lying every time, but perhaps misinterpreting evidence, looking at things the wrong way, not seeing it correctly, or any other in a string of possibilities why a person may be less guilty, or not guilty, of something they are charged with. It leaves plenty of room for me to argue things in most circumstances depending on what type of a case I have. I have learned that calling too many people liars will ruin my crediblity, so I am careful about branding people as such. But one person can be telling the truth and my client may still be not guilty, or less guilty.