I'll respond with my favorite little (and big) gestures:
--Vito's face when he learns Sonny is dead, and when he uncovers Sonny at the funeral parlor.
--Vito's finger pointing to Tom to get Bonasera a drink.
--Vito dusting off Sollozzo's chair.
--Vito putting his hand over his heart when he pledges that he won't be the first to break the peace.
--Vito extending his hand to Bonasera to be kissed.
--Tom's grin when he hears Sonny banging Lucy.
--Michael's face after Sr. Vitelli invites him to his home.
--The way Roth's eyes glitter when he says, "This...is the business...we've chosen.
--Geary all puffed up when he thinks Michael is going to cave into his demands.
--Neri, after Michael asks Rocco and him to step outside so he can talk to Tom alone, starts going out, then stops to swipe a cigar.

...and about a hundred others.