Since we're talking poetry here, sound is all important in creating an overall feel in "Fra Lippo Lippi." And that overall feel is one of a rambling drunken man coming home from a night of partying hardy. Of course, that's exactly what Fra Lippo is up to this evening.
First of all, the poem sounds conversational and colloquial. Check out the "Form and Meter" section for some further deets on how the blank verse form contributes to this. It's not just the form that creates this, though. Browning's word choice and punctuation also play their roles.
He keeps things colloquial (meaning like a typical fifteenth-century conversation might sound) by adding in words like "Zooks!" (gadzooks, a mild curse word), "Boh!", and "Ouf!" These replicate how a man might talk in a conversation among friends, which takes us out of the more artificial feel that much poetry can give.
Browning's punctuation also keeps things on the conversational side. Did you notice all those dashes and ellipses? Check this out:
His name is Guidi—he'll not mind the monks—They call him Hulking Tom, he lets them talk—
He picks my practice up—he'll paint apace.
I hope so—though I never live so long,
I know what's sure to follow. You be judge! (276-280)
Each one of those dashes represent not only a pause, but a shift in the speaker's thoughts. If you think about it, a normal conversation with friends proceeds in fits and starts. You don't just smoothly present a monologue that is 100% pre-written and goes off without a hitch. You make connections, backtrack, add on, etc. And that's just what Browning has Lippo do here. It makes it sound much more true to how a real conversation might proceed.
But, the guy's also drunk. Browning captures this in some of the rambling that his speaker does. He feels the need to tell his entire life story to the guards, and also lets spill some of his own secrets—namely about how he's not able to stay away from the ladies and climbs out his window with a sheet ladder. He also adds in fragments of popular songs of the time, showing how Lippo loses his train of thought and sings melodies. We're sure you can totes picture your own friends doing during a normal conversation.