Character Analysis
The first thing you need to know about Fezzik is that he's an absolutely huge dude, and the second is that he's really strong. But because he's so big and strong, people always assume he's dumb, which is something that strikes Fezzik as unfair. He says:
"People are always thinking I'm so stupid because I'm big and strong and sometimes drool a little when I get excited." (5.51)
Now to be fair, Fezzik isn't the smartest guy in the world, but he's as loyal as they come and a good (and useful) friend to men like Westley and Inigo. After all, where would those guys be without a giant around to smash through any door or soldier they come up against? And despite his beef with people dismissing his brain based on his body (something Buttercup also experiences—read up on her in this section for more on that), as the novel continues, we learn that Fezzik is really insecure in moments where he's called upon to do some actual thinking:
When it came to power, nothing worried him. When it came to reading, he got knots in the middle of his stomach, and when it came to writing, he broke out in a cold sweat, and when addition was mentioned or, worse, long division, he always changed the subject right away. (5.158)
His best response to these moments of stress is to create rhymes in his head. As the author tells us, "Anything you said out loud, he rhymed it inside. Sometimes the rhymes made sense, sometimes they didn't. Fezzik never cared much about sense; all that ever mattered was the sound" (5.644). Inigo Montoya drinks to handle his difficulties in life, Fezzik makes rhymes. To each their own, we suppose.
So at the end of the day, Fezzik is basically a soft-but-deadly teddy bear. He's great if he's on your side, but downright scary if he ain't.