People had heard of Bruce willis though, GOTG had no huge names among main characters. The trailers for GOTG had alot of music and energy in them and really if people didn't like the movie even with the Marvel tag on it the movie wouldn't have made so much money.
Zoe Saldana? Bradley Cooper? Vin Diesel?
Zoe Saldana is hardly a big enough of a name to be a meaningful box-office draw all on her own. And while Bradley Cooper and Vin Diesel could be box-office draws on their names alone it hardly counts when they're only providing voices (and mo-cap) and their voices are distorted in post or they're "doing voices" to the point you cannot even tell it's them. (Neither of them sound like themselves) and, as pointed out, Vin Diesel only really has one line (two if you want to count the "We're" over "I'm.") Granted he puts a lot into his one or two lines and makes each one mean something different but it's still one line with him doing a voice. Pretty much anyone could have done it. Same with Bradley Cooper, a lot of good performances in his line reading but that's it and, again, he's "doing a voice" so you can't even tell it's him.
Sometimes with animated movies a big-name can be a draw when they're providing a voice but usually in those cases the person doesn't "do a voice" and sound like themselves.
That leaves us with Pratt and Saldana as our leads to attract audiences and neither of them quite have the appeal to do it all on their own, though Pratt probably does now given his performance and physique in the movie. At the very least his star is rising. Not sure Saldana has much going for her, really, and this is also a case where makeup sort of conceals her.
As said, no one was going to this movie to hear Vin Diesel grunt "I am Groot" or to hear Bradley Cooper rant like a psychotic raccoon. No one was going to this movie to see Zoe Saldana in green make-up (or Kare Gillian bald and in blue make-up, for that matter if we want to get wider in box-office draws) or to see Chris Pratt strut around to 70s music.
The entire cast is replaceable as far as talent goes. They all did great jobs but it's hard to say they all owned the roles or were a draw. Unlike, say, Robert Downey, Jr. -with all of his faults at the time- being a huge draw for Iron Man. At the time he was till more-or-less an A-list actor, or at least high on the B-List with an asterisk after his name saying that he can't easily be insured and to keep your eye open for mirrors and credit-cards with scuffed-up edges.
This movie was a risk for Marvel. Anecdotal, but for most of the past Marvel movies most everyone I knew was going to it opening weekend for Guardians? No one was sure because the trailers just didn't sell them and the movie looked ridiculous with the tree/raccoon characters (as well as the soundtrack for the trailer. I liked it but I admit it gave the movie an unusual-looking tone.) But after seeing it and telling people about it, I got many to go see it and everyone liked it the.
Marvel was certainly confident people would like the movie and word-of-mouth would get people in theaters post pre-screenings and the first weekend but people were unsure of this movie before it was released and before word-of-mouth spread.
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