Tools of Characterization
Hamm may allude to "Hamlet," making Hamm something of a raving prince who will ultimately bring about his own destruction. (Read more about Hamlet here.) It may also suggest the "ham," someone who is loud, silly, and outgoing and constantly wants to be the center of attention.
Clov brings to mind the word "cleave," to tear apart or rend, as well as the word "love." Taking the two of these together, we get the sense that love has somehow been torn from Clov.
Nagg's name raises the verb "to nag," as he is constantly whining and asking for different things from those around him.
The significance of Nell's name is unclear, but it is the most mellifluous-sounding name, and Nell is also the calmest character.
This type of wordplay may seem silly, but Beckett was particularly aware of the different associations his words would take on (remember, Beckett was a minimalist), and there are probably plenty more that are not listed here.
This play being what it is, we might better classify this section as "lack of actions." Yet, we learn a lot about Clov from that opening scene where he draws back the curtains of the windows, but keeps forgetting the ladder when he does so. Through Clov's actions, we get a sense of his absent-mindedness and also of his absurdity. We also learn a lot about Clov by the way that he constantly goes back and forth from his kitchen. He is incapable of actually leaving Hamm, so he substitutes in a bunch of these false departures. We get into the significance of the kitchen at more length in the "Setting" section.
Because Hamm and his parents are immobile, we don't learn as much by their actions. Yet, we do notice that Nagg lets himself out of his bin and goes back into it as he pleases, whereas Nell waits for someone to call her or put her back. This might suggest that Nagg still possesses more agency and hope then Nell does.
We also notice that Hamm wants to be taken around the room frequently, and that he attempts to move himself with the gaff, the poll used for sailing. He is not content being physically handicapped, and has not yet given the hope of being in control of his own body, no matter to how small an extent.
Habit is not a huge factor in the play in terms of specific behaviors, but it is very clear that each character has a set routine and is then driven forward by inertia. We do not, for example, see many people breaking habit. Now, Nagg has the habit of asking for different items of food, which suggests that he is needy. He also has the habit of yelling repeatedly for what he wants, which makes him seem childlike. Hamm has the habit of taking off his toque (hat) and putting it back on in salute. This might suggest that he still wants to maintain some sort of personal dignity by making such gestures. He also has the habit of calling Clov when Clov is right next to him, which underlines how afraid of being alone Hamm is. Clov, for his part, has the habit of constantly forgetting things, which seems like the only spontaneous, pleasurable action he really takes during the play.
Thought and Opinions
Because so much of the play takes place in dialogue (which makes sense, of course, because this is a play), we learn the most about characters by what they say and do. We do not count this as "Speech and Dialogue," however, because what defines the characters is not so much their speech as the ideas that underlie it. Hamm, for example, is characterized by his thought that things may not have been for nothing, that the grass may be greener on the other side. He is also characterized by the way that he thinks of himself as a monarch, and by his general hesitancy to end his own life or his interactions with those around him. Clov is characterized by his conviction that things are very nearly finished, and that everything is pretty much meaningless. Clov, unlike Hamm, longs for the end.
Though Nell is a very minor character, we can't help but characterize her by the speech that she makes about nothing being funnier than unhappiness. Nell understands black humor; she gets it. She explains it better than anyone else in the play, but she is just too exhausted to laugh any more. She admits that she has given in to despair. Nagg, for his part, is the least brainy character in the play. He is dominated more by his needs and desires than by anything else.