Perhaps the first time he struggled with the word "truck" he reapproached it by repeating "fire" and it worked, and then in subsequent scenarios he still expected to have trouble with it, so it gradually took more and more repetitions to think through to get ready to say the "truck." If he stops saying "fire" the stutter becomes a 'block,' an unintentional pause which makes it even more difficult because the harder he tries to push the word "truck" out, the harder it is to say it. (Getting comfortable with that silence and with stuttering itself was a big turning point for me. There are plenty more loops and catch-22's in struggling with a stutter but I think this is one that most of us are familiar with.)