Using Hamlet by William Shakespeare as a reference, support this quotation: "Human beings are defined by what they do, not by what they think."
I'm not so sure the premise of this quote is correct, as we all tend to do things after we first think about them; if that is the case, it is the thinking which shapes our doing. Since this question is connected to William Shakespeare's Hamlet, the point of the question is undoubtedly to characterize Hamlet by what he fails to do rather than by what he intends to do; however, both his action and his failure to act define Hamlet.
It is an interesting and easy thing to talk about Hamlet as a character who suffers from the inability to act; while it is true that he generally fails to act in the one area that matters to him (avenging his father's death), Hamlet should be considered a man of action.
As a man who has committed himself to avenging his father's death, Hamlet vows to put every other thought and intention out of his mind until he is able to get revenge, and he does quickly formulate a plan of sorts. From this intent, however, there springs very little action. He hesitates to kill Claudius many times for many reasons, not the least of which is the fact that Hamlet will forfeit his eternal soul if he kills a king. (He feels the same about taking his own life: see his first soliloquy). We should not fault Hamlet for taking his time to make sure he is not somehow being deceived into killing Claudius by a ghost or for being unwilling to kill Claudius when he believes Claudius is confessing his sins. It is true that his inaction for most of the play ends up causing others, including himself, to die; however, Hamlet does, in the end kill Claudius. In this case, we can choose to define Hamlet as a man of thoughtful consideration, a man concerned for the afterlife and for the consequences of murder because he does not immediately kill Claudius.
Thus conscience does make cowards of us all;
And thus the native hue of resolution
Is sicklied o’er with the pale cast of thought;
And enterprises of great pith and moment,
With this regard, their currents turn awry,
And lose the name of action.
On the other hand, we can certainly also characterize him as a weak-minded man who refuses to act despite the preponderance of evidence which indicates Claudius did, indeed, murder Hamlet's father.
Let us also consider the actions Hamlet does take in this play. He is very quick to repay his former friends, Rosencrantz and Guildenstern, for being Claudius's spies; when he has a chance to do nothing (he had the note and could just as easily have "lost" it) or have them killed, he does not hesitate to write and sign their death notice. Hamlet has just passed up an opportunity to kill Claudius before Hamlet goes to his mother's bedroom; however, when he hears someone hiding behind the tapestry, he is quick to pull out his knife and stab the man he thought was Claudius. It is the wrong man, of course, but Hamlet took action when he thought he could catch Claudius in the "sin" of snooping. What this says about Hamlet, of course, is that when he is pushed and has thought about it enough, Hamlet can be a man of decisive action.
Again, defining people by what they do is also defining them by what they think, presuming that people act on what they think. Hamlet is a man of both action and inaction, and both of these things happen after Hamlet has thought about it. Perhaps thinking too much is the cause of Hamlet's inaction; however, it is also ultimately the cause of his actions (i.e., when he has thought about it enough, he does act).