A classic example to consider when answering this question is how at the end of Act I both Willy and Biff talk about Biff going to Bill Oliver to ask for a loan to start a business. This is interestingly paralleled with Willy's resolution to go and speak to his boss, Howard, about having a desk job rather than continuing to work as a travelling salesman. Looking at this part of the play, it is fascinating to see how long both Willy and Biff talk about these ideas and how they discuss them in such intimate detail. Willy's imagination clearly runs away with him, and before Biff has even made contact with Bill Oliver, Willy is already thinking about how much money he can get from Oliver:
I see great things for you kids, I think your troubles are over. But remember, start big and you'll end big. Ask for fifteen. How much you gonna ask for?
The audience gets the sense with this heated and serious discussion that Biff and Willy enjoy talking more about what they are going to do than they actually enjoy doing the act they are talking about. Both have put off their plans for some time, and now that they are on the cusp of speaking to Oliver and to Howard respectively, they seem to invest more energy into thinking about doing it than actually doing it.
When this fact is placed alongside some of the main themes of the play, the audience understands that this is merely another tactic adopted in order to ignore reality. At heart, Biff (perhaps not his father) knows that his success with Bill Oliver is less than guaranteed, and yet he and his father are happy to talk about the deal as if it is already something that has been successful. It is as if they prevaricate about taking purposeful action to prolong their delusion that little bit longer, as they at least subconsciously recognise the foolishness of their dreams. Of course, for both of them, the reality comes crashing home when they actually take purposeful action and their dreams are destroyed.