ROMEO Give me a torch, I am not for this ambling;
Being but heavy, I will bear the light.
MERCUTIO Nay, gentle Romeo, we must have you dance.
ROMEO Not I, believe me. You have dancing shoes
With nimble soles, I have a soul of lead
So stakes me to the ground I cannot move. (1.4.11-16)
If Romeo has a torch then he won’t be able to dance (or amble as he describes it, slightly dismissively; ambling is moving lightly, easily, or perhaps like an old person, or even effeminately). He’s maintaining his pose of the outsider melancholic, unable to join in the pleasures of the evening. He’s still capable of poking fun (and puns) at himself, though: his melancholy makes him heavy, so he’ll carry the light (a familiar quibble); his heavy, leaden soul is contrasted with light dancing shoes, with their nimble soles. Romeo is stuck, and he knows he’s stuck.
And here’s Mercutio, sidling into the play at last. He’s friendly and polite, like Benvolio solicitous of his friend, calling him gentle (but the fireworks will get underway shortly). He’s not named until later in the scene, but Romeo’s invocation of lead can be contrasted with Mercutio as mercurial, like mercury, quicksilver, definitely not melancholy (the melancholy humour is governed by Saturn). Mercutio makes things happen.