By Act III, Banquo has become suspicious of his former friend. He tells the audience as much through his words as follows:
Thou hast it now: king, Cawdor, Glamis, all, As the weird women promised, and, I fear, Thou play'dst most foully for it;
He, too, received prophecies from the witches and knows that Macbeth's have come true, and very quickly. While he hopes that Macbeth has not "helped" the prophecies along through murder, he cannot rid the thought from his mind. His following conversation with Macbeth unfortunately gives Macbeth the time and occasion to kill him before he can voice his fears and save his own life.
Banquo's very short answers to Macbeth's questions regarding if and when and where he intends to ride that day seem to suggest that he is trying to give Macbeth as little information as possible, and this could betray his growing suspicions of the king as well. When Macbeth asks if Banquo is riding out today, Banquo literally just responds by saying, "Yes, my good lord." He offers no additional information about where or for what purpose he is going.
When Macbeth presses him for additional information, asking if he's riding far, he replies, "As far, my lord, as will fill up the time / 'Twixt this and supper." I can't escape the feeling that he is being intentionally vague, saying only that he's going far enough that he'll be gone all day, but he neglects to say where, exactly, he is going. Then, when Macbeth asks if Fleance is going with him, Banquo essentially says, "Yep, gotta go! Buh-bye!" It feels as though he cannot escape Macbeth fast enough, and he is clearly not anxious to share any additional information about his activities with his former friend. This helps to show his growing suspicion of the king.