How can I fetch the tags of the selected page?

    label: Select Page
    type: pages
    multiple: false
    subpages: false
    help: Automatically fetch the sub-pages on the page you select.
    query: site.index.template('default')
    width: 2/3

    label: Selected Page Categories
    type: select
    options: query
    query: page.selectPage.categories.split

When the user selects a page, I want to get the categories for that page. Is that possible?

Nope, only after saving.