Frequently in Beowulf, the narrator saves you a lot of trouble and pesky critical thinking by just telling you what someone is like. For example, at the very beginning of the epic, the narrator gives a history of several kings of the Danes, starting with, "So. The Spear-Danes in days gone by / and the kings who ruled them had courage and greatness" (1-2). There's no guesswork here: we've been told directly that the kings of the Spear-Danes are courageous and great. Of course, the narrator will go on to explain why, which is where the next tool of characterization, actions, comes into play.
The ultimate test of a medieval warrior is what he does. Can he put his money where his mouth is? Or, to put it another way, can he put his sword where his boast is? Beowulf becomes great through his larger-than-life deeds: defeating Breca in a swimming contest that involves both endurance and sea monsters; fighting the demon Grendel in hand-to-hand combat; battling a fire-breathing dragon that scares nearly all his followers away. But what's really important isn't succeeding in these actions – it's just trying. Medieval warriors were keenly aware that, sooner or later, they'd be killed in battle or in the attempt to do one more heroic feat. What makes them heroic is that they do these great deeds anyway, despite the fact that it will eventually kill them. Beowulf doesn't have to live through every battle to be a great warrior; in fact, it's even more awesome if he eventually meets his doom.
Although actions are important in the world of Beowulf, interestingly enough they don't always speak louder than words. What do we mean? Well, most medieval warriors have to be their own publicists. When Beowulf arrives in the land of the Danes, he introduces himself with an elaborate speech describing his prowess and skill as a warrior. He's not ashamed to blow his own trumpet, so to speak. He stands up in the middle of Heorot Hall and says,
"I battled and bound five beasts,
raided a troll-nest and in the night-sea
slaughtered sea brutes. I have suffered extremes
and avenged the Geats (their enemies brought it
upon themselves, I devastated them).
Now I mean to be a match for Grendel." (420-425)
There's no false modesty here! Beowulf isn't going to wait for others to praise him or for history to vindicate his name, because a great Scandinavian warrior boasts about his own great deeds. In fact, boasting was one kind of formal speech that kings and warriors made at this time, and how well they boasted defined part of who they were.