The items are what The Guide uses to steal the emotion/energy from people. When Astrid makes them leave willingly, the energy goes to "God" since that's how Purgatory works. So she can't talk to them normally if she's going to help The Guide.
You can view it like this: The Guide notices that throughout the different timelines, Astrid has a talent for helping people deal with their emotions. So he gets the idea to have her be the one to try out the items, and it works.
The items are all tailor-made for each person (the painting symbolizes Locke's longing for his family, the empty book symbolizes Rahel's conflicted mentality etc), and when a powerful/sincere emotion is triggered close to the item then the person is drained and their energy is put into the item instead, so that The Guide gets it instead of "God".
Oh, and you're welcome! I love talking about my games so this has just been fun, I'm happy that you're interested in wanting to know more. :)