Why does Montag have trouble moving his feet as he approaches the firehouse? What does Faber remind him?

Montag has problems moving his feet because he is afraid of making mistakes. Faber reminds him that mistakes can be profitable.... that mistakes can teach lessons.

"My feet," said Montag. "I can't move them. I feel so damn silly. My feet won't move!"

"Listen. Easy now," said the old man gently. "I know, I know. You're afraid of making mistakes. Don't be. Mistakes can be profited by. Man, when I was young I shoved my ignorance in people's faces. They beat me with sticks. By the time I was forty my blunt instrument had been honed to a fine cutting point for me. If you hide your ignorance, no one will hit you and you'll never learn. Now, pick up your feet, into the firehouse with you! We're twins, we're not alone any more, we're not separated out in different parlours, with no contact between. If you need help when Beatty pries at you, I'll be sitting right here in your eardrum making notes!"