Originally Posted by dyna mo

But how exactly did Walt poison Brock?

The complete timeline goes something like this:

Walt is sitting in his yard with the gun, waiting for Gus?s men to kill him. He sees the Lily of the Valley plant on the patio. (5:30 into Episode 512)
Off camera, Walt crushes up some of the poison from the Lily of the Valley plant and injects it into a juice box.
Also off camera, Walt delivers the juice box to Brock?s school. (Having seen Brock the day before at Jesse and Andrea?s place.)
Walt puts the juice box in Brock?s lunch. (Again off camera)
Walt calls Saul, and has Huell lift the pack of cigarettes from Jesse. (More off camera)
Huell, Saul?s bodyguard, lifts the pack with the Ricin cig from Jesse, and replaces it with a clean pack with one cigarette missing. (This can be seen in Episode 412 if you know what you?re looking for)
Walt lies to Jesse about all this in Episode 412 about this and Jesse believes him and is now willing to help him kill Gus.
(For his part, Bryan Cranston hadn?t read the Episode 413 script so he, too, believed that Walt was telling the truth, making it easier to give a convincing performance.)
?The way we worked it out, [Walt] had just enough time to do it but it would have been very tricky indeed. It was improbable perhaps, but not impossible,? said Gilligan about the missing backstory. ?But he was a very motivated individual at that point.?