Walter tricked Astrid in order to inject her with a sedative in order to steal a bullet-shaped metal cylinder at the behest of the Observer. (The Arrival) This placed a tremendous strain on their trust. Walter later apologized to Astrid, who did not reply.
Astrid was willing to risk her life to stay in an infected building, to help Walter save Peter. (What Lies Below) In doing so, she demonstrated tremendous faith in Walter's ability to synthesize a cure for Peter. She talked a despairing Walter through the process of creating a cure, giving him enough hope and inspiration that he was able to successfully create a cure.
- Walter': (to Astrid, hours after he had injected her with a sedative.) What I did to you was very untrustworthy of me. I never intended to harm you. I was simply doing what was necessary to protect us all. If it would help you feel a sense of retribution, I would tell you to inject me too. But... but I'd most likely enjoy it.
- Season One, The Arrival