Characters not mine.
(Originally written for a three things challenge on comment_fic. Prompt was "Three little words with one big meaning.")
There are quite a lot of things Sirius Black is good at saying. Most of them are insults, or smart remarks, or the sort of thing you say to a girl when you're drunk and not really interested (because he's never really interested). There are a few pieces of smalltalk in there, too, and the answer to whatever question McGonagall has just thrown at him to see if he and his friends were actually paying attention.
But for all the things Sirius is good at saying, there's at least one other thing he can't force past his throat. For all the insults he throws at his mother, he stumbles over apologies to Mrs Potter or Mrs Lupin. For every smart arse remark there's something honest he could have said. And for every tipsy flirtation there's a word in the dormitory he didn't say. Sometimes it amazes him that Remus and James have never seemed to mind.
The reason James lets everything go unsaid may be all in the word "brother," of course. James has never had one before, and he's had to take his cues from fiction and friends and Sirius himself. And back before James, back when Sirius looked at Regulus and saw a brother instead of a stranger, "family" meant parking in the middle of Reggie's bed in the middle of a bull session or smacking him upside the head so he yelled at Sirius instead of Mum, and "I'm sorry" meant letting Regulus pin him to the ground in the middle of a fight instead of the other way round. And maybe James sees things the same way, sees "family" in the muddy pawprints on the sheets and hears the apology in the lack of retaliation when James finally looses it and hexes Sirius purple.
Remus is different. He's sly and still a little nervous and neither Sirius nor Remus are entirely certain where they stand with each other. Remus doesn't ask for anything; he's as afraid as Sirius is that this will all disappear. But Sirius doesn't want lies or flippancy and raw honesty is hard when you're not sure of anything. So Sirius is left hoping Remus knows what he means when he hugs the other boy just a little too hard, or teases him mercilessly about his romantic tastes when there's no one else around. He thinks Remus does.
As for what he can't and won't say? Mostly, he's convinced himself they're just words. From him they're just words.
From James, "Let's go home" means more than he can ever pay back.
And from Remus, "I love you" carries a kind of permanence Sirius wishes he were that sure of.