In the book, Rinzler tells us that McGregor took the scene seriously from the outset, asking everyone there to “be still” because, “It’s a very important moment.” George Lucas gave him some direction, saying, “As you watch Anakin slide down, how about if you take one step forward. For a moment, you think about it. Your first impulse is to save him — but then you realize you can’t.”
That moment, when we see Obi-Wan make the decision to leave Anakin there is just heartbreaking. This is where Obi-Wan realizes there’s nothing he can do for Anakin to help him, and it rips him apart knowing that he had to make the decision to walk away. The lines Obi-Wan says after Anakin yells, “I hate you!” were then changed to reflect that. Originally Obi-Wan says, “I love you. But I will not help you,” and Lucas explained that what he’s really saying here is, “You were our only hope-and you blew it. Now we don’t have any hope.”
Lucas’ direction was to make it all softer and change the tense. The book explains:
“After Anakin implores Obi-Wan to save him, George asks Ewan to say ‘I will not…’ softer, almost to himself … ‘After he bursts into flames,’ Lucas directs, ‘it’s as if you’re talking to a dead person. To a piece of toast.’ He suggests, to drive home this point, that McGregor change the words in the script to the past tense, ‘I loved you.’ The actor acquiesces, but points out that his subsequent line would have to change to, ‘But I could not help you.’ Lucas agrees, and [Jayne-Ann] Tenggren [script supervisor] alters the script accordingly.”
