How is Banquo represented as being honest and open with everyone?

First, when Banquo and Macbeth receive the Weird Sisters' "prophecies," Banquo's response is characteristically artless.  Although Macbeth seems to believe them without question, and is further convinced by the fact that their first statement -- that he will become Thane of Cawdor -- has come true, Banquo is open about his concerns.  He says,

But 'tis strangeAnd oftentimes, to win us to our harm,The instruments of darkness tell us truths,Win us with honest trifles, to betray 'sIn deepest consequence. -- (1.3.134-138)

He worries that the Weird Sisters may be attempting to do the men harm by telling them the truth about something small so that they will believe in the inevitability of something bigger.  Specifically, they might have told Macbeth the truth about becoming Thane of Cawdor just so he would believe that he would also become king; then, if this belief ignites Macbeth's ambition, it could lead him to do things that would be harmful to himself or others.  Banquo openly shares his concerns with his friend from the start.

Later, at Macbeth's castle, Banquo admits that he "dreamt last night of the three Weird Sisters" because, to Macbeth, "they have showed some truth" (2.1. 25-26).  Their words, and perhaps his own suspicions of Macbeth's capabilities, have been weighing on his mind, and it seems like he's trying to sound his friend, to figure out how much Macbeth is considering what they said. Macbeth, characteristically deceptive, says, "I think not of them" (2.1.27).  Macbeth has been thinking a great deal of them!  Shakespeare is developing Banquo as a foil to Macbeth.  A foil is a character who contrasts with the protagonist in order to highlight particular qualities of the main character.  Banquo's honesty emphasizes Macbeth's willingness to lie.

Even when Macbeth suggests that if, in the future, Banquo will support him and join his cause, it will benefit Banquo, Banquo says that he will do nothing that would force him to lose his honor:

So I lose [no honor]
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counseled. (2.1.36-39)

Banquo will not consent to do anything that might go against his conscience, and he is honest with his friend, even though such a statement might convey his suspicions to that friend.  Why would Banquo think that Macbeth might ask him to do something that he would find unscrupulous unless he felt fairly certain that Macbeth was capable of committing unsavory or immoral acts? 

With Banquo, what you see is what you get.  He is unafraid to stand up for what he knows is right, even if it could upset a friend.  He is unwilling to betray a friend in order to win the favor of another. 

Once Macbeth has committed the murder, Banquo "fear[s] / [He] played'st most foully for 't" (3.1.2-3), but considers the possibility that since what the Weird Sisters told Macbeth came true then perhaps what they forecast for him would come to fruition as well.  He speaks his hope once and then says, "But hush, no more" (3.1.10).  Perhaps he doesn't even want to consider the possibility that his progeny would be kings because he doesn't want to be driven to violence the way Macbeth has been.  Or maybe he knows that for the crown to pass to his family, something terrible will have to happen to Macbeth.  Either way, he stops himself from dreaming because of the possibility that it could lead to further violence.  This is an honest and good man, indeed.