As I said it's about the effectiveness of the screentime used and the context in which it is used.
Quote
-- INTERIOR: VADER'S STAR DESTROYER -- BRIDGE -- CONTROL DECK
Vader stands in the back control area of his ship's bridge with a motley group of men and creatures. Admiral Piett and two controllers stand at the front of the bridge and watch the group with scorn.
PIETT: Bounty hunters. We don't need that scum.
FIRST CONTROLLER: Yes, sir.
PIETT: Those Rebels won't escape us.
Bossk, one of the bounty hunters, looks down from his perch and growls at Piett. A second controller interrupts the moment.
SECOND CONTROLLER: Sir, we have a priority signal from the Star Destroyer Avenger.
PIETT: Right.
The group standing before Vader is a bizarre array of galactic fortune hunters: There is Bossk, a slimy, tentacled monster with two huge, bloodshot eyes in a soft baggy face; Zuckuss and Dengar, two battle-scarred, mangy human types; IG-88, a battered, tarnished chrome war droid; and Boba Fett, a man in a weapon-covered armored space suit.
VADER: ...there will be a substantial reward for the one who finds the Millennium Falcon. You are free to use any methods necessary, but I want them alive...
Vader walks to a stop in front of Boba Fett and turns to face him.
VADER: ...No disintegrations.
BOBA FETT: As you wish.
Here we see Boba Fett being singled out as, we can assume, a hard-arse who is prone to disintegrating his quarry. This one detail helps to flesh out this whole group of 'Bounty Hunter scum'.
It's true that Bossk doesn't do anything much in particular to advance the plot but in the context of the scene and by virtue of being included in a group of Bounty Hunters with Boba Fett he becomes more than a mere archetype and is given a small 'moment' which layers his character. Granted it's a thin layer but it's still thicker than any of the paint-by-numbers prequel characters.
An audience doesn't need characters to spout of reams of dialogue about who they are and where they come from and what their motivations are. That kind of information is all well and good for the actors to build the foundations of their characters on and extremely useful in helping film novelizers to fill the page count but all it does to the movie is bloat it like a three day old corpse in a summer stream.
My point is there's no difference between the methods used to realise the characters of Bossk or Chewie. Given their respective screentimes we know enough about each to move on with the story. Whereas expository dialogue slows the film down to a halt and neither endears the characters to us nor the screenwriter.